are all power conditioners noisy?

I have an API Power Wedge 116 - hums.
I complained to the mfr and they replced two of the three isolation transformers. It helped tremendously but guess what? It still hums - just not as bad.
I also have the Monster HTS 5100 - hums too.
If it has a transformer it is susceptible to humming!
Variations in the inpregnation of the windings will allow this to happen. Mfrs. pull a vacuum during impregnation but, to cut corners, they don't pull a vacuum as long as is really necessary to get all the air out and so occassionally, you'll get humming. Grrrrr!
At one time I owned two Parasound A51 power amps at one time - one hummed noticeably, the other did not - go figure.

I would follow the other suggestions of trying to damp the transformer/housing and tightening screws. At least you will kill the hummming transferring to the housing which is probably the bulk of the annoyance.

Perhaps you have DC voltage from the outside transformer that is comming into your lines. This will make a transformer HUM in a power conditioner and or a transformer in a amp.

You can check this with a Digital Volt meter yourself or have the power company check this.

If you don't need electrical storm protection, the Mapleshade Records (home of guru Pierre Sprey) top model power strip actually acts as well, often better, than many traditional power conditioners. It just looks so homebrew despite helping a system very much.
Aside from that, my own [never-opened to tighten] Powervar is dead silent in another location where it is used.

BPT units are DEAD quiet and offer balanced power in all kinds of doses. Best unit I've ever used was the BPT 3.5 Signature.