What brand/output? Did it ever work without buzz? Try cleaning the input & output RCA plugs that plug into the reverb unit. If not RCA plugs, check for swollen capacitors. Replace swollen capacitors.

Unplug the guitar. Does it still buzz?

If it does, the problem is in the amp. If it doesn't, suspect the cord/plugs/sockets or guitar wiring.

Does it continue to buzz when you play? What kind of guitar are you using?

Some amount of amp buzz for a connected but not playing guitar in an overdrive setting is not unusual, especially with a cheap guitar or one with single coil pickups.

Hey sorry really late response. Thank you guys for responding though! So it's an older Fender 212R Frontman Amp (It's 2 x 12" I was wrong before). My cords are definitely fine, nothing to do with the pedals, I've got a decent Yamaha Bass I've been running through it and that's not the problem (at least I'm pretty sure of all that).

It's definitely something to do with the amp I think. If them bass is plugged in and the amp is on then it's fine, a reallyyyyy low hum maybe but you turn up the reverb and get an unpleasant and very noticeable white noise. I'll try cleaning the input / output RCA plugs. The reverb is built into the amp. Where do you check for swollen capacitors? Also, I run the amp clean (no dirt).

Are you confusing buzz with hum? They usually have different causes.