Help with a loop/hum question, please?

Here's the deal. My effects loops. When I am running some pedals into my effects loop off of my board I am getting some hum - not too loud but noticeable. When I yank the send, the hum goes away. I tried this on two different amps with two different kinds of loops, I've changed power sources and even just ran on batteries, I've changed all of the cables (and double checked the connections on each one - what a pain!) Nothing works for eliminating the hum, EXCEPT, instead of using the loop, taking the identical cables/powers sources/everything and plugging the return cable into the amps input and plugging the send into any guitar. Is what I M experiencing simply the effect of running line level (is that what it's called?) effects into a loop? Is this why loops should be dedicated just for rack or rack compatible gear? If so, I guess it's time to run everything out front.........

Thanks for your help!!!