If it’s been fine up until now and you’ve not made any other changes then I doubt additional earth wires will help. I’d start by checking the Rega arm cable RCA connectors are good and tight on the input sockets on the Superline.
The Ground terminal on the SL is really just for connecting the arm cable earth wire. On the Rega, this is done via one of the arm cable shields. It’s just strange why the buzz has just started if you’ve had no problems before and have not made changes to the system.
If you’ve checked the RCA connections are clean and tight (unplugged and re-attached) then I’d power the system down and check all the connections between Superline, SC and 552 to make sure these are ok.
Just one other thing to mention, your profile mentions you have a Naim CD player and Naim streamer. What’s the ground switch set to on the streamer ?
My suspicion is as per James’ post above - the next-gen sockets on the Superline only have a very small contact area and sometimes the surrounding soft plastic can make it hard for a non-next-gen RCA Phono plug to make a solid contact. With the locking plugs that Rega use you need to move them around a bit with the collars slackened off then tighten up as much as you can. Any poor contact will give you a buzz as, IIRC, Rega earth through the negative of one of the leads.
I would disconnect and try again.
Also note that if you have moved anything or added anything, the Superline is very sensitive to transformers nearby. That also includes any tonearm cables running alongside or near mains leads, or connected SNAICs.
Thankyou James and Richard. I am sorry I have wasted your time. I happened to notice on the front of the 552 both buttons for aux1 were depressed and lit. I selected another output and left input at aux1 and buzz went. I guess some for of feed back.