Hi all,
I've got an engine sound in my 94 Altima (with 178,000 km) that I'm trying to identify. It's actually two sounds that I think are caused by the same thing.
First, at idle, when the car is warm, there's a sort of a gentle knocking or pulsing sound that happens about 4 times/second. Sometimes it seems harder and louder than others. Also, I don't hear it outside of the car, even with the hood up. I don't hear it inside the car with the hood up either, but there's a bit more noise with the hood up and a resonance I don't get with it closed that might be masking it.
I loosened the main drivebelt to the point that most things weren't turning, and ran the engine warm for a few seconds, and the sound was gone. I thought it was probably something on that belt, but then I took the A/C belt off (with the main belt back on) and couldn't hear the sound again. I'm wondering if it might be related to the crankshaft somehow... because of the stress on the pulley maybe some play at the end of the crank that is being exposed? I had the crank seal replaced twice recently (without realizing it, I had forgotten about the first time) and I'm now wondering if it's getting worn prematurely because of the movement of the crank.. is that possible? Or would I have bigger, more obvious problems if that were the case?
The other sound happens at higher revs, I'd say between 1000 and 3000 rpm. It's sort of a fluttering sound. It seems to be worse in damp weather, and it's more noticeable in cold weather or when the engine is cold. It seems like it's more related to the air temperature than the engine temperature though, ie in the winter on cold days, it's still a lot more obvious even when the engine is warmed up than it is in the summer. I only hear it during medium-light acceleration or when holding a constant speed, at full throttle it's not obvious, but it might just be getting drowned out. I've suspected it might just be the vibration of the engine making the exhaust shield resonate, because it sounds almost like the sound I get if I tap that shield with something metallic, but I'm not convinced that's it.. I'm a bit paranoid I guess.
It could be these are two different things causing the sounds, but in any case, I was hoping for some thoughts in case anyone recognizes these symptoms. I might try to record some samples of the sounds I'm describing and post a link if anyone wants to take a listen.
Thanks in advance.