Rash on one of my male breeder Rats

Today is cleaning day and I was moving some rats around when I noticed that one of my large breeder males has what looks like some sort of rash or something, its hard to see it really but I noticed it when he turned and I could see under his coat some, I watched him for a bit and he is scratching some but other than that he seems ok. Just to tell you a bit about my setup, I have a small rack system for them, there is plenty of floor space in the tubs, never over crowded, I use Aspen bedding, I clean once a week, they have plenty of water and food..not sure how it could be my husbandry, they are in a climate controlled room...well thats about all I can think of..any ideas?

Can be a few things, food allergy , excessive grooming, fight , parasites.