2004 V6 RPM drop then rebound when letting off gas

Hello all, I've searched everywhere and cannot seem to find a solid answer. She's a 2004 4Runner Sport Edition 4.0l with about 215k miles. While driving When I let off the gas the RPM will drop and then rebound back and then slowly drop like normal. The RPM drop is accompanied by crackling/rumbling from the exhaust. I don't feel anything when it happens. I did replace my muffler and reroute my Tailpipe recently, I don't know if I just barely noticed because of the louder exhaust or if it just barely started.

 

Your ECU throwing any codes?

Probably recommend cleaning your MAF sensor and throttle body for good measure. Maybe also you or someone check your fuel supply line and make sure theres no obstruction/drop in pressure when there shouldnt be

 

Thanks for the reply! ECU threw a P0420 and a P0430 due to leaking at both exhaust manifolds gaskets, replacing both manifolds today. Only other was a briefly pending misfire in cylinder 1 that went away almost immediately and hasnt returned. MAF sensor is brand new, throttle body got cleaned at the same time, also tried new spark plugs, new coils, and new intake manifold gasket. Just running some fuel system cleaner for the injectors.

 

^^^ THIS.

Cheers,

Sean

 

Well I put one new manifold on, but It looks like the gasket wasn't the only leak point. There is a huge gap at the flange right before the Downstream O2 sensors on both sides. Could this be the culprit? Maybe when I let off the gas some air sucks in or something?