Don't tell the electrician... show him.
If he has no clue of what to do to fix that, get another electrician.
of the best paths to follow, apart and along with the above posts, try to eleiminate the loop yourself. Tons of info here on how to do that very thing... and I've posted to them till I'm blue so search for ground loops and go from there.
The primary sources are cable boxes, and satellite boxes. Sometimes phone systems that use a DC power sup backup too.
OK.. just flip off all breakers except for the ones supplying the audio gear. turn them on one at a time and check for the buzz.
if with all the ckts off at the breaker panel save for the audio and you still get the buzz... lift the cable ground. Wait 15 min or so and check it.
If the buzz is still going with all ckts off but the stereo system ckts, and the outside cable box ground is lifted, it's one of two things.... it's one of the pieces attached to those ckts feeding the stereo, OR... as was/is my own case... check the inside of the breaker panel and see if your ground bus and neutral bus are tied together.
In some areas, this is code. So fi they are and the electricain OKs it... I've found only one other solution.... well two, but only one cheap one.
Lift the ground wire from the outlets feeding the gear.
Mo' 'spensive one:
Transformers. Isolation transformers specifically for the dedicated ckts. this is a boon too for the system as it aids to a lower noise floor. A dedicated ground wire other than your service ground can them gbe used.
It's a simple enough thing to do, and the aprts aren't the costly things, it'll be the labor to put them in that will hurt more.
I lifted the grounds on every ded ckt except the one I use for the preamp & Processor. I also used 'cheater plugs' on verious appliances using 110v, dishwasher, fridge, and WM.... but that's me.