Look guys I'm sorry for bringing this back but holy s*** I really need help this is driving me NUTS.

Things I've done:
Fit cat-back exhaust with new resonator
Tightened the S*** out of the flange at the bottom of my header
Removed all heat shields from cat, header and above resonator
Looked around for anything loose under the dash
Applied new silicone sealant to the header gasket (which is still good)
Lubricated the belts pulleys
Found that the top timing cover was missing the two bolts that hold it in, and put new ones in
Looked for anything loose in the engine bay, there was nothing
Asked mechanics and none of them had any idea

Yesterday I handed my car over to a workshop that specializes in japanese cars for them to have a go at fixing it, they told me they adjusted the tappets and checked all the belts and things in that area and around the timing belt, apparently the idler pulley was a bit loose and the bearing was noisy, so they replaced the bearing and tightened it back up. (although on the receipt it said "R&R A/C tensioner pulley and replace bearing") after this they said they couldn't hear it anymore, and this morning when I drove my car I couldn't really hear anything, but tonight I noticed it was there again, only now instead of starting to buzz at 3000RPM it's doing it at around 4200RPM and is a bit quieter. So whatever they did it affected it somehow. I'm not sure if it went away and came back after a bit of driving or if I just didn't notice it till tonight.

I've been told that sometimes it can be the tensioner pulley for the timing belt, so I bought a new one and supplied it to them, however they told me there's no way a tensioner would fail till around 200,000km on the clock, and when they finished the work they said the tensioner seemed fine.

When the sound begins around 4200RPM it's starts like "bzz.. bzz..bzzbzz.." and then 4800-5000 is just "bzzbzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz" all the way up the rev range. And you can't hear it from outside the car or even if you stick your head in the engine bay and rev it.

I've been told many times by mechanics and others not to worry because it's not affecting the engine at all, but I know its not normal! and I really wanna get rid of it!

ANY help would be so much appreciated and if there's anyone in ACT who could come look at it please tell me!

stats on my car incase it helps:
DC2 VTi-R 1995 B18C2
ODO: 134,XXXkm
Doesn't burn oil, smoke at all, and no power lag or anything.
I've been very nice to the car, never had my foot to the floor in the 4-5 months I've owned it.