A couple days ago, my Civic started making a low vibrating noise when I accelerate under 40 mph. It sounds kind of like a dirtbike or a lawnmower. I looked at it a little bit tonight. It's definitely coming from somewhere underneath the car about in line with the front door, not the engine compartment. I'm thinking it might be a heat shield by the exhaust, but it's dark now. I was wondering if anyone else on here has had this issue and if it sounds like it might be anything more serious. (I really hope it's not the transmission. I checked the fluid levels and they were fine.)

 

I to have this issue now. I changed my manifold and throttle body and it started doing it out of no where. it did it through 1st-3rd gear now only in 1st gear.

 

check if anything is lose underneath as you said doesn't sound like a tranny problem off the bat.