rattle knocking after warming up from under car

[airman]

Just picked up a 1998 aurora and I notice that after warming up there is a loud rattle knocking noise from under the car near the catalytic converter area. It is not there when idling and more so when under load or with quick engine rev ups. The noise does follow the engine RPM. Just drove it over 400 miles none stop with no problems and all indicators are good and no sign of exhaust smoke (oil 40psi, rpm ~21k @ 75mph temp 200). Can't hear it at hwy speeds. Most of the 400 mile trip outside air temp was over 100F. It also seems a bit weak on power than my last 99 aurora. Everything else seems good???

[tigger]

Sounds like your cat is starting to break up inside. Get the car up on ramps or stands and give the cat a couple good smacks with the palm of your hand. Wait until it's cool or wear good gloves, haha. Should give you a pretty good idea of where the rattle is coming from...

21,000 rpm at 75mph?

[airman]

More like 2400rpm at 75 and oil 50psi. I don't get it. Everything else looks good. I'm taking to the shop Tuesday.

[RCA1186]

I second what tigger said. Same thing happened to the cat on my '99. Honeycomb broke up inside and would rattle around with engine revs, most annoying sound ever. After I replaced the cat it went away. If it's plugging up your exhaust it could explain a lack of power too.

[rorafan]

Can't remember if our cars have a metal heat shield above the converter or not...could be that also, if you're lucky (much cheaper). They sometimes come loose and rattle against the cat/exhaust. It can be quiet until the exhaust moves enough to make contact.

Had the honeycomb breakup issue on a mid-90s Blazer I used to own and yup, super annoying sound. It was def. a contributor to lack of power in that case also. When you banged on the converter it was clear what the problem was. Sounded like marbles in there!

[Jorge]

I have the same problem w my 95...Idk..what else to do..at first I thought it was the ac compressor clutch..but I'm in doubt now...willing to spend the money on a new catalytic...i just want that rattle away!