I thought about mentioning that (N1/2 tacho) I have had similar on a 727 which only cleared by accident when the Tacho Generator was changed for a different defect. Couple of days later smart pilot announced that the noise had gone !
I decided that it wasn't relevant in this case and didn't know what Lycoming did for RPM indication,could be like a car. I have never been involved with anything with less than 80 seats !