Hello,

I bought a clean 99 GM last week with 89k miles. The car seems to run fabulously except this strange issue. BTW it's the dual exhaust version.

I was going to pass someone and when floored the car made a very noticeable rattle sound that quickly goes away once the gas is let off a bit (only when floored). My question is where should I look first could this be a catalytic converter issue? I looked under the car and it has an H pipe but I saw zero downstream 02 sensors???? The other explanation I was thinking is the AC compressor rattles at idle but I've never heard it cruising around.

Any thoughts would be very helpful, I bought the car cheap and don't want to go around throwing money at it (that's what the Mustang is for

Sean

 

What octane of fuel are you running? Sometimes (tho' rarely) my 95 will suffer pre-ignition under hard acceleration with regular gas.

 

It has just made the same exact noise after putting in 91 and seafoam in the tank. Last night I cleaned out the MAF well so I'll see what that does. I'm thinking it may be a vacuum leak that only really affects the engine at higher RPMS. One of the vacuum lines to the intake plenum is cracked in numerous spots. I also plan on checking the PCV

Sean

 

Idle Pullys, Check the BB's
Mercurys are known for it, I live in FL, and every merc I owned needed them serviced.,
Very keep and When your at WOT the motor shifts to HIGH RPMs and then then the pullys are the most STRESS.. You did say rattle, the CATS Air flow under High RPM would Become Silent, since the air flow is keeping the junk from bouncing around at a Stand still the cats will rattle some, if the filters are breaking up.