Not sure if this helps, but where I work we have a piece of machinary that has a "waste trap" under it. This get full of mud/water etc and it has to be cleaned out all the time - this gives off a really strong Fish smell (stangnant water / mud mixed up) - there is nothing else that goes through it that causes the smell so I assume it's to do with the mud/water standing. Could the ground be very wet and doing the same kinda thing in your run?
