strange: wet-dog smell?

I started noticing a mild wet-dog smell coming from one of my males' cages. his testes dropped recently and I want to keep him for breeding. But why is he smelling like this! He seems healthy in every way and his cage is clean and dry (I have him on shredded aspen.) I am not feeding or watering him any different than my females or other older male. Any thoughts? It is not overpowering, but it is kind of icky, as I hate this smell..

I had two girls that gave off the "wet dog" scent. A black, and a grey and white one. All my rats came from the same place, but those two just stank!! They did have a wheel that they used ALL THE TIME. DAY AND ALLLLLLLL NIGHT. I took it out, and the smell went down. Then everyone was moved to a rack, and I haven't noticed the smell since. I figured it was "rat sweat". lol, from all the exercise they were doin. So maybe it will just go away like mine did. hopefully lol ~.^