I recently did some work for a local hair saloon. In one area exsisted one 2 lamp 4 foot wrap around I had installed the previous year. I moved that light and installed 3 more new lights, same exact fixtures. I turned them on and after about 10 min. one fixture started to buzz exsessivly loud. Figuring I may have a bad ballast I exchanged the bllast with an extra fixture I had purchased. The fixture started to buzz again. I replaced the bulbs and tombstones and it was quiet. A couple of hours later my customer caled me back to complain about the buzz. I replaced the ballast and the bulbs. Two hours later I got the call, Buzzing again. I replaced the entire fixture at this point but it didnt help .I called the ballast MFG and they told me it was normal without even hearing it and they suggested movng the ballast. so I turned the fixture around thinking I might be getting some resination from some hidden structural item. but to no avail the buzzing continued. By now my customer was highly anoyed with me and I was a bit frustreted so I purchased an electronic ballast fron a differany MFG.(originaly a magnetic ballast)
and that was the cure. The three other fixtures as well as 7 other fixtures I installed there are working perfectly. I am still puzzled by this. I hooked the fixture I took down up in my shop and it worked perfecty. I told my customer there was some sort of geographic magnetic anomoly in her shop so I could get paid. any thoughts on this would be highly appreciated.

Could it have been interaction with some other magentic-field inducing equipment in the vicinty, maybe on the other side of the wall?

I thought of that too. above the ceiling is the middle of a living room floor for an appartment

Sounds like the Magnetic (Linear Reactor / Core and Coil) type Ballasts were vibrating the heck out of everything that was connected to the ceiling.

Loose fitting fixture hardware is a major contributor to the problem. The problem is compounded when a Mag Ballast with sound ratings greater than "A" are used.

Hard lid ceilings with no dampening material (insulation, etc.) - especially when the upper part is also covered, are really great acoustical amplifiers!

FYI: Some Electronic Ballasts may incorporate Linear Reactors (AKA "Hybrids"), and nearly all have Mag Amps (Saturable Reactors) which are magnetic, however, unless the output frequency is low (or has a really powerful harmonic/sideband/overtone/etc.), there shouldn't be any excessive and noticable hum.

Ran across some thin Advance Hi Hz Electronic Ballasts for F32 T8 Lamps (upto 3, solid 430ma output, or normal power) which hummed lightly. Must have been the AC input filters, or a very strong overtone from the output reactors!

Scott35