My 2000 Galant has started making a rather loud noise from the engine that sounds like something slapping against plastic. Initially it only did it for a couple of seconds when I first started the car and I thought maybe an acorn had somehow gotten into something. Now it does it all the time except when I'm driving and the engine is above around 2,000 RPM. In other words, I can drive around town with overdrive off and I only hear it when I slow down for stop lights. It's much louder than a lifter tick and doesn't really sound like any internal engine noise I've ever heard but it does vary in frequency with the engine speed.

I've checked to make sure the belts aren't frayed, including the timing belt which I replaced about a year ago. With the main belt removed, it still makes the noise. I just got the valve covers removed and will be continuing work on it tomorrow. I'm just looking for ideas as to what it might be and what I should check. It's likely coincidence but it really started doing it the night the temperature dropped here, but the engine temperature has no effect on it. It has the V6 engine by the way.

 

I realize this is a very old thread but it may be useful to someone on the forum. The noise you are experiencing is most likely from the timing belt slap due to a failed timing belt tensioner. I have just experienced the same sort of noise on my vehicle this evening - loud slapping sounds like on plastic. The noise is worse at idle and goes away once you are around 2,000 rpm. I was able to diagnose that it was not coming from serpentine belt and not from any accessories. The next thing was the timing belt. I pulled off the covers and found a very slack timing belt. The tensioner is definitely toast - has oil leaking from the push rod part of it and I can compress the rod 4-5 mm just with my fingers (it should move less than 1 mm with about 150 lbs pressure compressing it). This btw is on a 3.5 l V6 on a '01 Montero. I was lucky that the engine stayed in time and did not jump a tooth. Will be ordering parts and do the full timing belt job in the next few days.

 

This is indeed an old thread. I don't remember all the specifics but I had to do a lot of work to the car after posting this. I do recall the tensioner was very rigid. It was also missing a clip or something on the back of it.