edd - do you get easily annoyed about things?
just kidding, but no it must be iron deposits or something similar. I wouldn't worry, look on the bright side, I have to do a full service on both our systems this week sometime.
Wiring, plumbing, checking floor of vans for damage etc, blah blah blah - I know if it ain't broke, don't fix it but I like to keep an eye on everything.