Appears to be the motor mount. Put the car up on ramps, had a friend hold the brake and give it gas, still heard the sound. Put a jack under the oil pan and transmission pan (with boards in between) and jacked it up a bit. No noise. In drive with the brake depressed, I lowered the jack and as the motor sat down it started knocking again. Gonna pull the mount and replace it tomorrow, will report back when it's replaced if this fixes the issue.