What is this noise? (engine-on noise from front subframe area)

Otter74 · 04-23-2020, 05:22 AM

I de-hibernated my car a few weekends ago, and last Sunday my girlfriend and I took it grocery shopping. This was the second time I'd driven it this year. On the way to the store (I can walk to my neighborhood store, but obviously I needed to drive to the suburbs to go shopping

), I noticed an odd noise. From the driver's seat it sounded sort of like an HVAC noise, sort of a hissing, coming from somewhere near my feet. Noise was present with engine on, vehicle can be moving or stopped. A bit puzzling because there really isn't much going on in the front of the car. The only energy source I can think of (other than chemical energy, of course) is the fuel pump. When I stopped for gas, I got down on the ground and tried to capture it on audio. It's the higher-pitched sort of rhythmic sound you can hear over the engine noise.

jlex · jlex's Garage

Can't hear much. Could it be a footwell blower motor?
Or maybe the blower motor for the front A/C condenser...

Otter74 · 04-23-2020, 05:39 AM

Yeah, that was the best audio I could get (with my phone). It's the periodic sort of hissing sound. Car has no AC and backdated heat, so HVAC is all passive.

Pedro '84 Coupe · Pedro '84 Coupe's Garage

Fuel pump noise shouldn't be periodic when the engine is running. From what I could hear it almost sounded like an exhaust leak, but that shouldn't make noise at the subframe area. If you suspect the fuel pump, remove the protective plate and get under there while the car is running. A mechanic's stethoscope or listening through a length of rubber hose may help you pinpoint the source.

yelcab1 · 04-23-2020, 05:59 AM

You can turn off the engine, apply 12V to the pump and see if the noise appears.

proporsche · proporsche's Garage

if you open your engine deck lid.and hear this sounds better? sounds like a dry alternator belt..lets see the video from the engine ...
ivan

Otter74 · 04-23-2020, 11:11 AM

I'm going to turn the car on and have another listen this evening when I get home. From the driver's seat it can be heard very clearly - as clearly as when I'm under the car - which suggests it's something being transmitted through the body structure. Those can be tricky sometimes because sounds can travel quite a ways and hide their origin. My Saab had a rattle that sounded like it was coming from the cargo area that turned out to be the exhaust hitting the Panhard rod bracket. Haven't listened to it with engine lid open, so I'll try that.

proporsche · proporsche's Garage

yup..it would help, with the lid up...
Ivan

proporsche · proporsche's Garage

so? did you managed to make the new video??

Ivan

Otter74 · 04-25-2020, 03:38 PM

I did start the car briefly on Thursday evening and the sound is definitely coming from the front of the car, not the engine compartment. Forgot to make a video, though! (doh) I think I need to drive the car for it to be more evident, so I'll try again when I drive it Sunday.

rattlsnak · rattlsnak's Garage

turn key on/engine off and raise CIS sensor plate and see if you can hear it..

Otter74 · 04-25-2020, 06:15 PM

Quote:

Originally Posted by rattlsnak

turn key on/engine off and raise CIS sensor plate and see if you can hear it..

Car's running on a Megasquirt

rattlsnak · rattlsnak's Garage

ok, well, nevermind.. lol. Are you 100% sure its not the fuel pump then?

Arlo911 · 04-25-2020, 10:20 PM

Wild guess here, but you might want to check the vacuum line that goes from the tunnel up to the trunk where it connects to your brake booster.

Otter74 · 04-27-2020, 07:11 AM

Arlo, thanks - I'd never have thought of that. And it would account for something that pulses with engine rpm. However, when I drove the car on Sunday it was quiet Still, will check that line.

Otter74 · 03:30 PM

Update: since I filled the tank, the sound disappeared. Inclined to suspect a possible issue with the fuel pump. The tank was really empty - I don't usually run it that low. Will check again next time the tank is low.

Also, this is the engine from the engine compartment. On an entirely separate note, there is a low tone audible that I didn't used to hear.

proporsche · proporsche's Garage

put a long screwdriver to the alternator housing and the other end to your ear and listen ..possibly alt.bearing

Ivan

Otter74 · 05-12-2020, 07:25 AM

Finally put a screwdriver to the fan housing and I *can* hear a sort of jingly/rattly noise . I may have a shop diagnose this (there's a first time for everything) so I can focus my garage time on other projects that are better done by me.

Otter74 · 06-20-2020, 03:18 PM

Update on my last question, regarding the additional undertone to my engine noise. It's gotten more evident (and I took the car on a ~1500 mile trip last weekend) so I made another recording of it. It's definitely coming from the left bank, so I went from left to right to left to try to capture the contrast. Best I could do with my phone. In general terms, it sounds like an exhaust leak. Valves were just adjusted when I had a shop take care of some pre-trip items (first time I've ever taken the car to a shop) and no mention of head stud issues, and no other reason to doubt them (engine rebuilt about 25k ago, head studs replaced, I checked them a year ago and all good.) Have not had the chance to put car on stands and check for things like, well, an exhaust leak.