Thump/knocking under car when going fast

[sezo]

.
Now, i have a question about my new 99 z28 that i've bought, first car ever, so still kinda learning...i wanna see if anyone can help: Any time i step on the gas from a light, or just a complete stop, and go fast, i can feel/hear a couple of thumps/knocks under the car, around maybe where the y-pipe/exhaust is. When i'm on neutral and rev the engine, it doesn't happen...only when going fast from a complete stop. The knock/thumps are pretty loud tbh, and at first i thought it was the y-pipe hitting the ground, since it sags a little...but even on flat street level, it happens...it only happens when i step on the gas substantially, but at regular speed it doesn't happen. What do you think?

[transamtom]

Probably the transmission mount.

[sezo]

Probably the transmission mount.

Is this bad? Easy fix? hard and expensive?..

[SMWS6TA]

My money is on the torque arm bushing that connects to the tail cone of the transmission. If it is the transmission mount like Tom stated both are an easy fix. Just need a couple of stands, jack and replacement mounts.

I'd start with those 2 first.