Hello friends, It has been quiet from me for over a year now, but now I am back,

 The reason for this low profile in Beoworld last year is that I have been busy in building my family and me a new house.  Last wednesday we actually moved in and now I am starting to get free time to other things again . Free at last......

 Quite exciting to start to put my B & O ' s around in the new house.

I have a beautiful Beomaster 5500 White, together with the rest of the 5500 white series. As I now had placed it one of the rooms I noticed a humming/Brumm in it, even in stand by mod. The humming sound like 50Hz frequenzy and it is not coming out of the speakers.

What can this be and are there any cure?

Is the antenna connected to your cable-TV for its radio signal? Then it could be that you need galvanic isolation between the wallplug and your cable. I had the same problem with a Beomaster 6500, and the problem disappeared when I bought one of these.

http://www.dinside.no/840379/store-prisforskjeller-paa-galvanisk-skille

I believe it is the mains transformer humming. I have a 5500 that does the same.

Unfortunately I don't think there's any way to fix it (short of replacing the transformer and hoping the spare is more silent), but usually you can silence it enough by experimenting a little with the location. Move it a bit away from the center of the shelf, try another desk / shelf that doesn't amplify the vibrations, put some rubber feet under it, etc.

You may be right, since he says it's "not coming out of the speakers."

Is it possible to replace the mains transformer from a spare/broken BM5500 and then solve the problem. Has it been done by anyone?

Certainly possible... the real problem is finding a spare that actually hums less!

I believe the problem is the transformer core delaminating, or the windings gradually working loose. If you can find a spare 5500 that has spent 10 years or so of its life off mains electricity for some reason, it might be a good donor.

Try the simple things first.

The rubber bushings with which the transormer is mounted in the BeoMaster might be the problem. After a lot of years they get hard and don't do what they are intended for anymore, that is preventing hum from the transformer to be heard. So the pronlem might be solved by replacing these rubber bushings (8 in total, 4 on top and 4 under the transformer). Solved hum in a BeoCenter 9000 this way!