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
