O.T. Speakers static/humming

I have a Dell XPS 8500, with Windows 7 Professional, SP1,
with Spywareblaster, Malwarebytes, Avast , Windows Defender
and Windows firewall.

(1) TB HD
Intel (R) Core (TM) i7-33-3770 CPU @ 3.40 GHz
Ram 12.0 GB
System type : 64-bit operating system

I also have

I have a Dell Optiplex 780 Tower, with Windows 7 Professional,
SP1, with Spywareblaster, Malwarebytes, Avast , Windows Defender
and Windows firewall.

Intel (R) Core 2 Duo 2.93 GHz
4GB RAM, 750 GB HD
System type : 64-bit operating system

and (external hard drives)

(8500)
WD BLACK SERIES WD2003FZEX 2TB 7200
RPM 64MB Cache SATA 6.0Gb/s 3.5" Internal
Hard Drive

(780)
Seagate Desktop HDD ST2000DM001 2TB 64MB
Cache SATA 6.0Gb/s 3.5"
Internal Hard Drive


The problem concerns the 8500; I have an intermittent problem,
for some reason when I logged on today there's a humming/buzzing
sound coming from the speakers. This has happened several times
recently. I haven't done a thing other than logging on.

I tried restarting and disconnecting and reconnecting the speaker
connections but it doesn't seem to go away until I power off/on.

So what could this be?

Thoughts/suggestions
Robert