Strange Acer aux. speaker problem

Strange Acer aux. speaker problem. All worked good for awhile then about 1 month ago I started having intermittant loud speaker static. When it happens there is a very short burst and seconds later 2 very loud static noises in a row follows about 5 seconds a part. It may have 2 or 3 of these in a row usually about 10 min. a part, sometime 30-40 mins. a part. New speakers did not cure. This will happen even if the computor is turned off and the speakers left on. The only thing that works is unplug the speaker wire or turn off speakers. Will USB speakers take care of this or a sound card? Info below.

Acer disk top 64 bit (18 months old)
Onboard sound
Aux. speakers
Win 7
12 gb ram