Several microphones sometimes produce humming noises

Archived from groups: rec.audio.tech (More info?)

Hello,
sorry for writing that long and that confusing a text,
but I hope someone will be able to help me here.
I'm completely confused; how can the following happen
and what can I do in order to prevent it:

I've got two PCs, both running with Windows XP.
More than a year ago, I had connected a microphone
(referred to as Micro1 below) to PC1 which worked
flawlessly in those days. When I tested it a few
months ago, all recorded samples contained a loud
humming noise (see below for a more detailed description).
It was still audible the same way when connecting
Micro1 to PC2.

PC2 had been connected to another microphone (Micro2)
some time before that, which used to record samples
without any interferences, but added that humming
sound as well when I re-tested it some months later.

PC1 was connected to a new microphone - Micro3 -, using
which I could record without any humming noises. That
microphone was temporarily connected to another PC,
running with Win95 (about 2 months), where it seemed to
work, too. When I reconnected it to PC1, all samples
featured a the humming sound now.
Connecting Micro3 to PC2 now resulted in absolutely
clear recordings, without that hum.

During another test with Micro2 and PC2 as well as PC1,
the humming noise had disappeared there, too.
This was about 3 weeks ago. After not using it at all
for two weeks, Micro2 causes the hum again with both
PC1 and PC2. Micro3 now produces excellent recordings
on both PCs.

Description of the noise:
With the "Mic Boost" option being disabled, the noise is
clearly audible, but not too loud a low-frequency sound.
With "Mic Boost" activated, it's a loud noise which nearly
drowns (and maybe distorts) anything else spoken into the
microphone.

I always tried both microphone connections, front-side as
well back-side, of PC1; removed any other cables near the
microphone cable, closed all windows and doors and put
all mobile phones out of range.
Changing the recording volume results in the humming
noise being less loud, but those sounds that I would
like to be recorded are even less audible then ...

The microphones mentioned are nothing special; actually,
I got all of them for free with various sound cards or
software, respectively. I do not intend to record anything
of high quality, however, I'd just like to use the audio
communication as provided by MSN Messenger, ICQ and Skype.

I bet I'm not supposed to use a microphone with PC1 for
a few weeks, till they add that humming noise to any
recording, disconnect them for some time, and be able
to use them without any problems afterwards?!

Any clues to a solution? Thanks in advance :-)
Regards,
Florian Haag