Okay so I have a 98 honda civic hx auto. Just got the car recently when I'm driving its like the car rpm raises nd drops in like a humming matter. I've check out just about everything..
Does it have overdrive? If so trans could be toggling between drive and overdrive. Next time it does it take notice if you are on a slight hill or incline. If you are and it has overdrive then it's probably responding to the added drag from the incline and down shifting to drive and is probably normal. Also check your trans fluid level. If it's low the same thing might happen.
again- using only words to describe a problem doesn't supply the same info as feel and sound- could also be PCM related. PCM is adjusting mix based on bad or no inputs, is there by chance a check engine light on on the dash?
I had the same problem without the during driving but I unplugged the air flow and map censor till I had them replaced it dripped back down to a steady rev
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