2001 Toyota RAV4 rumbling/tapping

Within the last month or so when I start the car it has this low tapping or rumbling sound with some vibration that gets much worse when I put it into reverse. It isn't as bad when I shift into drive but it is still very noticeable. Seems to smooh out as it runs for a bit but is still there in idle. Quiets out a lot to almost nothing when in neutral and park. Seems like it gets worse or louder when gas is pressed. Transmission and known defect with ECM were replaced about 2 years ago. Not 100% about the timing belt but previous owner seemed good on maintenance so that should have been replaced already. Changed oil and spark plugs pretty recently and am about to change serpentine belt in hopes that this will fix the problem. Any help or advice is much appreciated, thank you.