I have a 1994 Prizm, 4A-FE engine. 107k miles. As I mentioned in a post awhile back I recently took it in for the timing belt change (overdue preventative maintenance) and asked the mechanic to also see about a vibration noise I was getting at idle. I thought it would just be the heat shield or something loose in the exhaust. Anyway he found two motor mounts were bad and replaced them. This solved the problem. Situation resolved -- I thought.
However, gradually the vibration noise came back, now 6 months later. This time I have the same guy look at it and he can't find anything, he took it to a nearby muffler shop and they couldn't find anything loose. He said he could replace the other two mounts and it would cost about $300-something for parts plus labor, but he honestly thought that were just fine as-is. So I took it to the Chevy dealer thiinking they might have more direct experience with the Prizm. The mechanic there did several tests including removing all the belts, etc, and determined that the noise had to be coming from within the engine itself or the transmission. He said he would not be able to tell for sure without tearing into the engine. He could only guess about a bent rod or camshaft or warped flywheel. Of course opening the engine and reclosing it will be big dollars labor. He said another mechanic in the dealership owns a Prizm that has a similar noise and he has just lived with it.
This noise is there at high idle speeds, but goes away at higher RPM, and there does not seem to be any effect on performance. Its there also if I am driving at 5-10 mph, such as driving in a parking lot. It is there in neutral and 1st gear (5 speed manual). It can be fairly loud and varies somewhat with outdoor and engine temperature. Since the noise is there in manual and 1st gear, I suspect its not the transmission (?). It seems to me if there was a bent rod or camshaft the vibration would be worse at high RPM.
Has anyone else run into this and what did you do to resolve it? If I just live with it, I suppose the engine could still last a many thousand miles, yet, as I had hoped. We were planning to replace my wife's car (150+k miles) first, soon, and continue driving this one for a couple years, yet -- cosmetically and otherwise mechanically its in great condition. I'm not one to rebuild an engine myself, so I think the cost of that is prohibitive compared to the street value of the car. Maybe I need a louder stereo.