Oh mercy, there are gobs of potential causes for that 'noise'! The part I would be concerned about would be that it's 'constant' rather than intermittent (if I were the repeater owners). Locally, we've been blessed with space on a commercial tower (non-paying) which is very nice. The bad side of that 'nice' is that one of the 'paying' users equipment is producing an unbelievable amount of 'noise' in the repeater. The not so bad side is that it's intermittent. The 'fix' is very simple, but it involves 'their' equipment, not ours, which is not so simple. that user has been 'advised' of the problem and 'cure', and we've been told it's been put on the "to do" list. Ever heard that before?? Oh well... Good luck!
Paul