Yet another "name that noise"
 

Hey everyone

I've got an 83 911SC with 104K miles that has developed a rattling noise on heavy acceleration (under load). I can only repeat it when the car is between about 4000 and 4500 RPM. It almost sounds like a loose exhaust baffle. Furthermore, there is no noise present when I rev through 4000-4500 RPM in neutral nor on normal acceleration when I'm shifting @ around 4000 RPM.

The last interesting tidbit is that the sound is not always there. I've only noticed it a half dozen or so times. It definitely sounds like it is coming from the engine area or exhaust...not from the tranny below/behind me.

Any thoughts on what it might be? Needless to say, it's hard to put a scope to it when I'm in the car driving under load. Apart from that the car is running beautifully.

Looking forward to your thoughts. Thanks.

If you can post a video or sound bite.

Unfortunately I'm not going to be able to get back in the car for a couple of days. Not sure how well my iPhone will pick it up, but I'll try when I get back. Thanks!

I would get under the car and check for anything loose, wheels too.

If it sounds like a playing card in bike spokes, it could be a broken head stud. Pull the valve covers and look for loose bolts laying at the bottom.

Does sound like rattling ball bearings in a glass jar?

Detonation, sound like a shovel full of gravel poured onto a tin roof?

Could be alternator bearings beginning to fail.

What was the outside temps and the temp gauge at?

Keep driving it, it'll get worse...ask me how I know.

Despite only having run a couple tanks of fuel through my car since it began, mine has been detonating since last December. (This is the gravel on a tin roof sound already mentioned).

Started as your describing, mostly noticeable above 4k, only under load. Then it got bad pretty fast (within 150mi of driving) where it was even pinging under high cold start idle and any type of throttle application other than idle.

I spent this entire season thus far (with the little time I have as I'm a dad of 2 under 2) working down a lists of possible issues. Even measure the AFR's which seemed ok, at idle (CIS adjusting) Did I learn more about my car? Yes Did I determine and solve the exact issue? No.

Which is why I brought it to the pros, dropped it off to see Dan Jacobs and the crew this morning.

Everyone...thanks for the notes. Sorry for not responding sooner as I've been out of the country for the last couple of weeks

In answer to the great questions:

Yes to gravel on a tin roof. Exactly the way to describe! No signs of pinging at any other speed or gradual acceleration. Also no backfiring.
I've already checked under the car. Nothing loose that I can find.
ambient temp when I've noticed it was around 28C /83F. Engine was fully warmed up...on the warmer side of the gauge. I never put a heavy load on the engine until it's fully up to operating temp.
I did recently run a couple of tanks of 87 through it as the manual says regular octane is fine. My next stop will be to go back to 93 and then probably a trip to my trusty mechanic vs trying to tweak CIS.

More to follow when I get it sorted out.

Thanks again!

Chain tensioners? Have you got retrofitted 'carrera' tensioners or the original ones?

good suggestion. I believe I recall seeing the receipt from the PO, but will double check. Thanks!

Even the oil-fed tensioners can fail.

It is easy to tell if you have oil-fed tensioners - just visually ID the oil lines running down the centre of each timing chain cover.

Even the oil-fed tensioners can fail.

It is easy to tell if you have oil-fed tensioners - just visually ID the oil lines running down the centre of each timing chain cover.

Great tips, thanks. I'll be back to the car after the weekend and will report on my findings.

Be willing to bet broken head studs that are rattling around.

Detonation from lower octane fuel (hopefully it's just fuel related).

Detonation is more like a ping instead of a rattle. Loos timing chain tensioners can also be the problem. But it that was the case the engine would run poorly.

Well... problem solved

CliclClickBoom, 911SauCy, and mreid nailed it!

Syphoned all the 87 Octane gas out and filled the tank with some 93. This is what I usually run. Took it for a 100 mi jaunt in high ambient temp (similar to when I experienced the problem) and the engine sounded and performed perfectly.

Thanks to everyone for helping out.