924S engine sound mystery

[notenoughcars6]

Hey,

I've seen several posts about strange engine sounds and people have gotten lots of help so I thought I'd give it a try as well. This sound just recently started. It's a faint (sometimes not so faint) wawawawawawawa sound coming from the engine compartment. I'll post a video link here:

https://youtube.com/shorts/GGusREaZ8jQ


Ignore the video as I just used my phone for the microphone. You can hear it slightly in the cabin but mostly when I walk around to the side near the battery. It seems to coincide with the belts/pullys and does get faster with higher rpm. Timing belt, waterpump, tensioners, balance shaft were all replaced about 5 months/ 500 miles ago. I used a mechanics stethescope and listened where I could (i.e. not around the belts) but I did listen to the ps pump and didn't seem to be coming from there.

Any thoughts?

[walfreyydo]

Could be a bearing in one of the timing belt/balance shaft belt pulleys starting to go. Were these replaced when the belts were done previously (sometimes this gets skipped)? You can check by removing the pulleys and spinning them with your finger/pinky inside the bearing. If you feel any resistance or roughness they should be replaced.

Another thing to check would be belt tension, making sure accessory/timing/balance shaft belts arent overly tight or loose.

Lastly something might be rubbing on the timing belt front/rear covers

These are the things I would check.

[wwdwgs]

I second walfreyydo.

[notenoughcars6]

Thank you. I am going to call the shop tomorrow that did the timing belt/waterpump service and find out what was and was not done. I was already planning on replacing the vacuuum hoses with the silicone vacuum hose kit this weekend so I will check the belts and pullys while I'm doing that. Thanks for the advice and I will let you know how it goes.

[Monkey Wrench]

the sound has a frequency.
mark the belts pit an obvioud timing mark on the crank pulley or waterpump pulley and a mark on each belt.

run the car and note which has a frequency that matches.

removing a belt can elliminate it as the cause you just can't elliminate the timing belt.

with the mechanics stetescope do similar, listen to a point close to the bearing for alternator , waterpump etc. anyhting that turns. note if any have sounds that coincide wiht the timing of the noise's frequency.
if you can see a belt run, watch it, is it fluttering or is it stable, the belts have joints and it is not uncomon at all to get one with a crooked joint that will cause a fluttering belt, so can a bent pulley. mialignment can cause sounds like that so put a long thin straight-edge up against the pulley ( engine off) is the belt running perpendicular to the pulley or is it running crooked? sight between the straightedge and the belt.

you could use a strobe that can be adjusted to the frequency of the noise then point it around, the thing that appears to freeze has a similar rotational frequency.

you know how a metronome is used in keeping a musical instrument in time when practicing? the metronome's frequency is made to coincide and acts as a guide, so the instumet player listens to that and that heps guide their frequency.. often a drummer guides th eband by providing a consistent beat for others to follow. a director may use a wand in a similar way.


this is quite similar, when you match the frequency of the sound you are probably looking at the cause. each belt and pulley may have its own unique frequency, one will likely match the sound frequency.

a belt that runs between two pulleys has 3 frequencies, each pulley has a different radius , thus it's own frequency and the belt also has a length.

id make a temporary visible mark so that as you hear the sound and watch the frequency of the mark, you may note one that has the exact same frequency , thats a clue. a hunk of white tape could do. just so you can see how fast it's cyclingwhile at the same time listening to the pulsations of the sound. the part producing the sound will have a similar rotational frequency.

if belts are jumping and not runing smoothly check the pulleys maybe one is bent or damaged. look at a point midway and study how well the belt is tracking , if it is "whapping", there is a problem there. if swapping the belt stops it from "whapping" then its a bad belt.

if you want you can ut a dial indicator on a pulley and turn it to check if it has a lot of runout.

a simple trick tape a felt pen to a stick, brace the stick against something solid, ever so slowly.move the pen near the pulley. when it touches stop. look at which side of the pulley you see the mark, thats the high side, do it again to verify it wasnt; just you being a bit wiggly wiht the stick. it should show the mark on the same point.. thats your high point. thats an easy way to not use a dial indicator. no measurements but yoll see if things have a warp or are running off kilter.

careful about the pinch points of the belt be very consious of anything like a loose shirt sleeve, long hair etc. apply common sense so you don't hurt yourself. belts can cause horrible injuries. guide yourself to stay safe.








you can take a belt off and feel for bearing roughness . as mentioned you can try adjusting the tension of belts, the one making the noise may change its sound or frequency with a different tension. if you can change the sound, you are perhaps isolating the cause.

belts pulleys and everything driven does change in frequancy along with the engine rotation speed. what would not? your heater fan or maybe an electric cooling fan.

just because you replaced, for example a waterpump, does not mean you may not have a bad bearing. one tiny grain of stray metal can easily destroy a bearing and factories are not always clean. if I get offshore parts. like a tpered bearing for example, Ill often wash them out in clean solvent, and repack them with clean grease, before assembly , that is, if they aren't sealed bearings. If I get offshore bearings, if they have the word china on them Ill often turn and just take oit to a bearing retailer, ask for a recognisable brand , they are normally only 20 bucks. Porsche never made bearings so OEM means little . most bearings are metric and of " standardized" dimensions. the number on a bearing or a seal usually is the clue to ID'ing it. google the number and confirm dimensions.. often there is a letter which identifys the bearing's seals, eg 2 RS may mean roller bearings with shields ,two sides. a bearing shop will usually give you choice of a numenr of makers and a proice range, for the same bearing. cheap bearings can cause bad surprises.

you can use something as a sound insulator and move it about , eg a cusion off th couch, pay attention to where it is located and what position seems to help block the noise from your ear. higher sounds are difficult to locate directionally lower notes easier to detect directionally. its not a high pitched squeal. the direction of the sound is a clue. you might do a test eg, is it from the right side of the engine compartment or the left side. is oneside louder when one side is covered or uncovered?

you can download a scope to yoru phone for free it can "listen" and show you frequency. as an example I found I can use one to hear sounds that are out of my hearing range.

I had a problem to solve it was an anoying high pitched noise that only younger people could hear. kids can set their cell phone notification sounds so only other young kids can hear it, people who are older usually have a more limited high range.

you may be able to move a cell into places you would't be safe to venture to with your human ear. the graph of a scope may be a way of quantifying volume or frequency.

the crank turns at engine speed, it's pulley's circumference is a fixed size.
from that and RPM you can calculate the belt's feet per second, with that and using other diamerters you can calcultate the speed of driven shafts EG: you can calculate the rpm of the waterpump based on sizes, belt feet per second, simple proportional math.

for example you can figure out the frequency, mathematically, of a driven shaft such as your waterpump based upon circumferences and RPM first find the belt speed then you can know the rpm of the driven shaft by factoring in its pulley size. if you like math you could use it to find what else turns at the same RPM as your pulsating "problem sound."