Permanent 50hz

@musicland1

Its often quite normal for an amplifier to pick up noise if nothing is connected to its input. Its also normal for the amplifier to buzz if the preamp is connected but not powered up.

If you want to test to see if the issue is the amp you could try putting shorting plugs on the amps inputs. Then power the amp on and see if the noise is still present.

If there is nothing connected to the speaker except power cord, and we are assuming it's a powered speaker, (no RCA or XLR signal cables) -- if you are still getting 50hz buzz, then I agree with Eric that the power supply is failing (probably a bad capacitor).  The amp/ps board in the speaker likely needs to be replaced.

actually the noise is quiet..i need to put my ear very close to the mid unit
from 5 cm I can't hear the rrrrrrr
i don't know if it's normal or not

Not normal and Why would a 50 Hz tone be coming from the mid range driver. The cross over should have taken that out. What kind of speakers are they?