Why is my new engine in my Blazer making sounds like popcorn popping when warm? on 2001 Chevrolet Blazer

Brand new engine, got it back Saturday. After engine is warm I hear popping sounds that sound like popcorn popping. Only happens after engine is warm. Also, can't hear it when hood is up, only when sitting inside. Doesn't matter if I am in park, drive, or reverse. Garage not being very helpful.

Asked by for the 2001 Chevrolet Blazer
Sounds like popcorn popping , hum,,,. Have the distributor, the dist cap and rotor checked to see, after it's warm and loosened up, if there is any interference. Will be more pronounced from inside the cab as well. However this usually sounds like it's coming from the heater area. Can usually feel it on the dist cap or listen to it with an auto. stethoscope. That's the only thing i know of that can cause that rapid, repetitive type sound in the rear of the engine other than maybe the flexplate/flywheel.
It does sound like it is coming from the heater. I originally thought it had something to do with the heater.
1 more answer , 4 more comments
Are you sure it's the engine? If you can only hear it from inside the truck, and not outside with hood up, in a garage. If it was your engine, you would hear it outside even better. Might start looking inside.
Yeah, I am sure. The reason I am sure is that when I rev it up, the sound goes away. When I let off the gas the sound comes back louder and faster and then goes back to being intermittent. I can hear it a little from outside. It seems to be on the drivers side toward the back of the engine.
Do you think it could be the convertor coming apart.
I don't think it is the converter. Doesn't seem to be coming from that area. Also, we just had new one (well, 2, is has 2 cats)and muffler put on 6 months ago. I am starting to wonder if it is and exhaust leak.
Prob need someone to increase RPM while another is under the truck to narrow it down.
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