I generally diagnose and fix my own problems, but this one has me stumped. I'd appreciate any input I could get
I have a 98 Eclipse NT and this issue started just recently

If I let off the throttle, I immediately hear a thud
If I drive at a constant speed, I hear a constant thud-thud-thud-thud
The sound only really goes away if I'm either accelerating or braking (or not moving)
I can feel the thud, but I hear it more than I feel it. It's not a violent thing, it's just obvious that something isn't quite right

I jacked it up and checked the suspension for play, and everything appears to be tight
In case it's related, I changed the timing belt a few weeks ago also, but there was no thump until a couple days ago. The Tbelt is tight and the engine runs/revs great.
Someone help please
Thanks in advance