Just A Theory
I can only offer a theory based on your description. If you disconnect the turntable and the buzzing disappears, you just found part one of the culprit _ it's the turntable. If it is the turntable, we need to find part two of the problem. First, most turntables have a ground wire that runs between it and the amp. Make sure that is connected and good. Most receivers, amps, and pre amps with phono inputs also have a ground connection. Connect the phono connections and jiggle them to see if the buzzing changes at all. If so, one of the phono connectors is bad. The other possibility is that the cartridge is not properly connected (seated) in the tone arm. Make very certain that you have a good connection there, because that is an area of frequent problems. If the problem persists, there is a small chance that the phono cartridge has "bought the farm" [rare, but it happened to me]. In that case, you have to change the cartridge. Sorry that I can't be more helpful without seeing it. Good luck.