Permanent 50hz

Is your speaker powered? If so it's probably a bad power supply in it.

You could also try a Hum Buster to see if it is you home wiring.
B

Given your 50hz comment I'm assuming you're outside the US. If the equivalent of 3-prong adapters exists for your local plug configuration, get one and see if the noise goes away. If it does, you have a grounding problem, which you'll have to work on component by component. If the noise stays, it's more likely your amp or an upstream component creating the noise.

Since you have said the hum only occurs with nothing connected to the amp I'm guessing the amp. I'm also assuming the sources are both unplugged from the wall and the interconnects to the amp. Is this a problem that just occurred or is the amp new to you? Have you tried moving the amp and speakers to another room or even a different location like a friends home or audio dealer?

@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?

If you have to put your ear to the speaker to hear any artifact its quite literally no concern. Almost any amplifier will have some audible artifact in that situation.
Don' worry'bout it.

It actually sounds like a minor ground loop hum.  If you have to get within 5cm of the speaker to actually hear it, then don't worry about it.  My system has the same thing.  Sometimes this will be very noticeable in systems and people can have a very hard time getting rid of ground loop humm.  Humbuster may or may not fix this.  Sometimes, you'll need to get the Granite Ground Zero to resolve this ($500+)

“Why would a 50 Hz tone be coming from the mid range driver?”

Excellent question.

The same reason midrange information comes through the woofer. Why do you think the woofer dust cap rings and buzzes like crazy?