[TenTec] Paragon Problem

[KA6UUP]

My 586 Paragon has developed a noise problem. It sounds like lightening 
crashes but lower in frequency.
It starts after the rig has been on for a while (1/2 hour or so) and 
will go away for a while if the rig is switched off and about a minute later
back on.
It is not affected by disconnecting the antenna.
It does not show on the s-meter but is controlled by the volume control.
It is present on all bands, all modes and all filter widths.
My plan is to get some cold spray and start looking for a noisy resister 
etc.
Any help or advice would be greatly appreciated
thanks,
Chuck KA6UUP