2001 mdx help help

this 2001 acura mdx i am working on has the crazy’s . If you are running alone on a flat road about 40 mph the tac will go from 1500 to 2000 rpm just back and forth and it makes a noise like a growling sound from under the hood some where but this is the only time it does it and there are no lights on. [any ideas would helpful ] trans fills like it shifts fine in all gears does anyone no if this is a common problem ? surly some one has seen this before??