Probobly DC on the powergrid. try connecting two power diodes in parallell butt in opposite direction paralelled with a 2200uf/35v cap. that should suffice it is very likely that something in your house or a nearby hose have something that puts a little Dc...
ignore the dots
Exactly. The noise is from the transformer which is most likely due to DC on the mains AC. You might need to filter this out by following the direct advice in the reply from hifi or carefully reading the link provided by Peter. All suggestions so far have given you the answer to your question.
Wessol, read the thread Peter gave and suddenly it will become clear...it goes in series with the primary of the transformer.