constant sound from speaker

[kizonis]

Hello,
I just received my M-audio BX5A deluxe monitor from repair, where they had to fit a new power transformer.
The problem is that since I plugged it in, the speaker emits a constant high pitched low volume sound which is rather annoying.
I tried to find the source of the sound and I realized that it is somehow connected to one of the two screws that connect the transformer with the back of the speaker. Every time I turn the screw a bit the sound stops and after a while it starts again.
Comparing the 2 speakers I realized that they had to add a small part, not sure how to call it in English but it is a small ring that prevents the screws from screwing in completely, something like the middle part of this http://
Any ideas on what I should do to fix it?
-the repair was not done by M-Audio but from a freelance technician and I could give it back to the guy but I am trying to avoid the whole process...-

 

[Saberus]

Tech messed up. That little bit of metal is called a washer. Take it back, point it out, and get him to fix it right (Or better, get your money back and go to someone else to fix it.)

 

[kizonis]

dont think I can ask for my money back, given that the guy ordered a new transformer for my speaker etc.
it wouldnt turn on at all and now it does work with this inconvenience.
I am trying to find the reason behind this sound and if it is something very simple, fix it at home.
there is also an update: I unscrewed it and screwed it again and now the other screw has the same effect...