I just swapped a 5sp in place of the auto and thats when the problem started. I hear this rattling noise that also feels like hammering. It always occurs during startup or engine shut down. It can occur at idle but won't occur until the engine is really warmed up. It won't occur while driving although if you idle down a hill you can feel it hammer a little bit. I have checked everything and I can't find anything loose and it drives fine. The cat has been replaced before but I did get a cat below efficiency code tonight. I tried shaking the cat but I couldn't really tell if it was making any noise. If I pull on the engine with my hands really hard when this is happening the noise will stop. Could my cat really cause this and be able to feel it while idling down a hill at low speed? Please help!

 

Is it possible the exhaust system is hitting the floorpan? When engine-braking, the engine rocks forward in it's mounts, which brings the exhaust system up closer to the floorpan.

Engine mounts loose?
What year car? Did you swap the vacuum-controlled rear mount with the one that's supposed to go with a manual tranny?

 

98 accord ex 4cyl. I didn't swap the rear mount and bracket, although it all bolted up fine. The MTX computer doesn't control the vacuum for the rear mount so I just hooked it up to vacuum all the time last night and it still does it. All mount are visually good. Whats wierd is that it only happens for a second at startup and shut down and when idling only when the car is hot and recently driven within 5 minutes. If I drive the car a few miles it will do it at idle but will stop doing it after a few minutes of just idling. Power is still good. Could the cat really cause this? I crawled under the car last night to check and all mounts are tight, everything is tight and I can barely hear the noise from under the hood but it is much louder inside the car.

 

Oh and I watched the exhaust while the car did this and the exhaust is clear of everything. The cat was replaced before I bought the car about 8k miles ago.

 

Well, if you tried with and without vacuum, and it didn't make any difference, then it sounds like that mount isn't the problem. Still, the manual tranny isn't supposed to have that vacuum-controlled mount at all.

The fact that it's loudest from inside the car, really suggests that SOMETHING is drumming against the floorpan.

 

I know thats just it but I looked underneath the car while it was doing it and the exhaust was hardly moving. When this happens the engine is hardly moving. I checked the clearance around the entire engine and transmission and everything is good. Nothing even comes close to hitting anything. I don't have the funds to pay a stealer to mis-diagnose it either. Its really weird.

 

If you can reliably make the noise by whatever means, then have someone else crawl around looking for the location of the sound. When you're sitting in the driver's seat, sounds locations can be very misleading.

 

I think I may have found the problem. I think it really is the rear mount. I pulled 2 plug wires to simulate the vibration so I could have a while to look at it and the sound appears to be from the rear mount. I must have tore it during a launch or something.

 

Ok I replaced the rear atx motor mount and bracket with a mtx one. First of all what a pain in the ***! But the problem is still there and I swear it seams to be getting worse. Cat looks perfect from the outside and still looks new and the rest of the exhaust looks good too including all heat shields.

 

Since you did just swap out the transmission, did you put on a flywheel from a MT car? MT CV joints? Use the tool to center the flywheel and clutch?

Just want to cover the basics, because you did a pretty big swap.

Make sure that you tightened everything back down properly especially the suspension. Check that CV joints are secure in the transmission and in the hub, etc...