Buzzing on a channel

Hallo!!!
I have a system made of nap 250, hi cap, nac 72. Now one speaker was buzzing. Inverting the speakers connections to the amp, the buzzing changed side. It means the speakers are ok. In your opinion, what could be the source of the problem?
Any hint?
Amp? Hicap? Pre? Cables?
There are ways to test and find out?
thanks cheers

Is it on all inputs?
What sources do you have?

I had an olive Nap 250 and XPS that buzzed. Both needed a recap.

How old is the NAP250? When was it last serviced?