Using American bought gear in Germany - unwanted hum/signal noise

[WCN]

I'm trying to use some electronics that I bought in the USA in Germany, and experiencing terrible hum issues when I introduce significant gain to the signal - i.e. several distortion pedals. All of the electronics I am using are rated 100-240V, so they are designed to be used with both American and European power supply. The odd thing is that I was staying at a house in Düsseldorf for a couple weeks and did not experience this problem. Now I am in an Apartment in Cologne, and when I chain more than 3 distortion pedals from the output of my sampler into a channel on my mixer, there is a great deal of noise and a very loud buzzing hum when no signal is passing through. Much louder than your average buzz/noise floor. Almost like a feedback loop. I'm guessing it's because there is an issue with the power supply to the building... but if anyone has any insight into this issue, and specifically how to eradicate it, I would greatly appreciate it.