I think you first need to find another shop!!!
If there is a noise when the clutch is engaged (pedal up) it cannot be the release bearing because there is no load on it. If there is a noise when the clutch is depressed, then the release bearing may be a suspect.
Most manual transmissions are subject to a phenomenon called "gear rattle", which is a torsional vibration in the gear train, and it's nothing to worry about. Also, the clutch release fork can rattle at idle, which can make an annoying sound, but it won't hurt anything. My 190E 2.6 five-speed manual emits an annoying rattle when the air conditioning compressor is on and it's idling in neutral. It sounds like a couple of ball bearings in a vibrating coffee can. I pointed this out to the dealer during the warranty period and after some discussions I accepted that it's a "built in feature".
Run some tests to see when the noise occurs. Idle, with/without the compressor on? Increase the revs slightly with the compressor on and off. Does it go away, stay the same or get worse?
Another possibilty is the clutch gear bearing - the bearing at the front of the transmission or another bearing inside the box. A little noise might be normal, but if a mainshaft bearing or countershaft needles are spalling, the noise level will increase as miles are accumulated, and it will be much louder in the lower gears when power is tranmitted through the countershaft than fifth, which is direct drive. It's something you need to keep an "ear" on. If it gets worse, it might be time to overhaul the gearbox.