Part throttle engine rumble during cruise

[jimtash]

Trying to figure out why I'm getting a loud engine rumble at highway cruise. During accel cars sounds fine, pulls to the redline with no problems but during normal cruise the thing makes this loud rumble that sounds like a jake brake on an 18 wheeler. I even turned the engine off while going down a slope on the highway and with the accelerator closed there was no noise but open up the throttle and it was there. I believe I could even hear it coming out of the throttle body itself. The engine is a lot harsher than it should be at cruise with this happening. Any suggestions.

 

[SHOdded]

I would suspect the timing tensioner, but I thought you already checked it out?

 

[jimtash]

It's not external to the engine. Another thing is that I hear crackling in the pipe as if something is hitting the inside. Wondering if the cats are coming apart on me. They are Magnaflows.