wrote:
Thanks all for responses.

It happens every time, after being off for a time, like over night.

My "Go" is the press of the start button.

I've checked out the 2 fans thorughly, and all is smooth, tight and
seems right.

The sound is hard to pinpoint 'cause it's so short and only happens
once a day, but seems central to the PC and I'm suspecting it might be
through the speaker, if there is one.

This has been an ongoing thing for some time now, but I've always
wondered about it, and occasionally put out out a feeler.

If you thought it was the optical drive, you could
unplug the optical drive SATA-power and SATA-data
cables, then start the computer and listen again.

On fans, pressing on the center of the hub may
modulate the sound effect, and help confirm
"you're touching the right fan". That's if the
fan noise was persistent, such as a bad bearing.

Some fans use FDB bearings (fluid dynamic). When
the fan first starts, the bearing is dry. After
a second or two, the FDB "pumping action" ensures
the fan bearing is coated in oil and then all
friction in the fan bearing stops. Perhaps
only where the fan bearing is held so the fan
can't come out of the bearing, that surface
could still wear, depending on fan orientation.
The very best FDB bearing man has ever built, has
been calculated to last 1200 years. They can be
good, as long as the lubricant never escapes.

The other bearing types are ball bearing and
sleeve bearing. Ball bearing, the bearing noise
goes up with time. The bearings continue to work,
even if the sound drives you crazy. (I have some
IBM 9GB hard drives that fit in this category
of being "too noisy" but "working fine".)

The cheapest bearing is a sleeve bearing. It's
like a ball bearing, only without balls. Typically
no seal at all. I got one sleeve bearing fan once,
where the oil had drained out and made a puddle
on the bottom of the chassis. Of course the fan
was destroyed in the first day of usage, since
the quality was a large -1.

A sleeve bearing will "rattle" rather than "spin",
when it's dry or has worn to an out-of-spec
condition. That's not your sound either, as sleeve
bearings could continue to make rude noises until
you rest your finger on the hub... for a while.

So if disconnecting the optical drive doesn't
provide relief, it's probably a fan. In a Dell,
it's the "hoover" fitted in the CPU area (the
one that vents the computer case, as well as
providing forced air through the CPU fins
at the same time). The Dell fan is normally
electrically turned down. In the event of a
control failure (Dell won't POST), the fan runs
full speed, because the BIOS cannot set the
register that controls the fan speed. The speed
is set higher, in response to measured temps
inside the casing. If there is no CPU in the
motherboard for example, then the fan should
default to high speed ("hoover mode") and scare
the crap out of you.

Paul