Pretty normal Bally behavior, floating the soundboard off the backplain grounds might reduce it somewhat.

Does it do this for all three coin switches? What are your dip settings for the three chutes?

replace old caps on sound board and check grounding

my guess you are using led displays, the buzz with them is extremely annoying, i may have had to add an additional ground going to the display loop or was it the 5v? can't remember but i did solve it

Are you sure the dip switches on the board are in fact set for Playboy?

It sounds like the games per coin is not set properly. Is this happening on both the left and right coin chute?
I have a B&I board, freeplay is set by setting the max credits to 35 or higher. I wanted my game to run in coin mode and discovered through trial and error max credits needs to be set to 30 or lower.
Why that is precluded by enabling 7 digits is unknown but admittedly I have not looked in the manual to confirm.

I would shut off the "same as chute one" setting and see what it does with the individual switches.

Another question- Was the MPU used in another machine before Playboy?

Go through the audits and zero out each one, some weird things can happen if there is old data stored in memory.

Still worth going through and clearing the audits in case there is some garbage data in memory.

i think after doing all recommended in this thread and still having the buzz, I used a couple jumpers and discovered with my machine if I added/doubled up on the grounds at the harness going to the displays the buzzing disappeared, i don't remember floating the sound board making any difference with my machine

https://pinside.com/pinball/forum/topic/bally-playboy-buzzing-sound

Old playfield switch capacitors have done strange activations for me. Disconnect capacitor to test. Not a bad idea to replace them all.

If floating the sound board fixes the sound it some kind of grounding issue. Bally isolates a lot the ground returns, when the sound board's backbox, 43v/12v and 5v grounds have any resistance between each other a nasty hum can happen. If it is working fine now I wouldn't try and chase down the problem.

There is kind of a bug in the software. Pop bumpers are active in all test modes for some games and I think it includes this one. So when one pop goes off, the one next to it is probably switch gapped to close and also fires with the vibration.

Bally games will be in free play when set to maximum credits, which is forty, unless the setup sheet has different info. What switches set max credits varies depending on what OS ROM is used, but it is specified in the setup sheet.

I typically zero out the RAM before sending out the MPU. Sometimes I will turn on the sound audit settings because I get a lot of emails about that. When switching to a new game, clear out the audits like this. Or use the RAM clear (combo rom dip 8 on, rest off).

Sorry, meant to say coinplay. I found that setting max credits to 35 or 40 the game remained in freeplay. I had to set max credits for 30 or less to get the game running in coinplay.