What is this low undertone in my engine sound?

Otter74 · 07-17-2021, 03:33 PM

OK, I think I’m going to try this again. This appeared at the beginning of the driving year and has been slowly becoming more evident. Occurs at idle, while driving, on the throttle, off the throttle, cold engine, warm engine. The sound I’m referring to is the low tone that you can hear underneath the rest of the engine noise. It is specific to the left bank. Video was taken on a cold start after a week or two of sitting. I don’t have a particular reason to suspect a broken head stud, as the engine was rebuilt about 30k miles ago with improved studs, and they were checked during a service a year ago when the valves were adjusted. No external visual evidence (e.g. soot) of broken studs. Exhaust leak also seems reasonable but I need a smoke tester to check that.

pmax · pmax's Garage

The idle sounds high but don't hear any "low undertone" of concern.

Otter74 · 07-17-2021, 09:22 PM

Quote:

Originally Posted by pmax

The idle sounds high but don't hear any "low undertone" of concern.

Yeah, ive struggled to record audio in which it’s evident. It’s really obvious to me in person, more so because I know what the car has always sounded like and it’s different, and only on the left side. Which, yes, is the exhaust outlet side.

Idle is high because I’ve got an intermittent too-much-air condition. Adjusted the throttle linkage and swapped out IACV to test and it’s not those.

proporsche · proporsche's Garage

one or two loose valve adjustments..that is what i hear

Ivan

Dpmulvan · 07-18-2021, 06:13 AM

I hear the low drone, could be a bad bearing in alternator or compressor if you have air. Try a stethoscope. Also hear what sounds like noisy valve tappets or cam chain. Impossible to tell through YouTube just where I would start to look. You can also use a screwdriver if you don’t have a stethoscope.

Otter74 · 07-18-2021, 08:45 AM

Thanks. Thought of the alternator a while ago and checked that with my ear and a pry bar on the housing, nothing unusual. Plus it’s coming from the left side. no AC, all that got ripped out. Valves did sound a touch loose to me but it quieted up and in any case loose > tight. I’m beginning to suspect an exhaust leak.

snbush67 · snbush67's Garage

I hear it, motors change, the valves loosen or tighten, rings can wear or stick, valves can crack or bend, all of it can change the tone. You could also check the motor and transmission mounts, could be a vibration coming from those.

High Life · High Life's Garage

Muffler issue?

Low drone could be a rotted baffle - hole in the muffler or exhaust gasket leak...

Otter74 · 05-23-2023, 04:55 AM

I thought I’d come back and follow up on a couple of things for the sake of the arvchives. The no-start that my car had (there’s another thread for that one) was diagnosed to fueling issues and during that process I diagnosed a bad injector on #4. Once the car was running again, it was still down on power, something which seemed like it had been creeping slowly in, just like the sound. Replaced the injector and all was good again - normal power, and no more wub-wub. Now, I did also replace a missing bolt at the flange of the crossover pipe to the pipe before the cat, but that had been missing for all eight years I’ve had the car. I’m chalking it up to a weird resonance due to the bad cylinder.