Resonating RPM

I just changed my spark plugs and I seem to have gained some lost HP. But when the rpm goes over 5 it starts resonating and the car sounds really wierd. I used the same type of plugs as in the manual. Any ideas?

Hey guy resonating? Check that the mufler covers are not loose. I've installed headders but left the OEM mufflers and the metal covers vibrate a little after 4500 rpms

Did you gap them to factory specs?

JoeJoe a misgapped plug will cause rough idle or stalling(in some cases) but not resonating. Have you seen otherwise? Please share with us

Well, when I answered this thread, I was thinking having it misgapped or not gapped properly at all could cause detonation.

Are you losing power over 5k bevah?

I think it could be from my new mufflers.

The sound is definetly from the mufflers and not the engine.

I tought so. Check that the metal covers are in place or take them off.
JoeJoe I do not want to be a pain :). Anyway my car had pretty much kr before installing the cooler plugs (6.6degs to be exact) and never heard a thing.
Have you had different experiences?