Every 6 seconds pop over the the audio tape out not on the speaker though.
It then has to be a capacitor in the tape audio line that are leaking, as the audio are taken from the speaker output and then reduced down in level by a couple of resistors and a capacitor or two.
Try to resolder the components around the tape out jack to see if the heat will change anything. Wasnt' there a schematic available somewhere? I think I've seen one somewhere on the web.
It could be the big capacitor in series with the speaker, if you somehow have disconnected the internal speaker, as some people do to their feed scanners to stop any local audio, and do not use a bleeding resistor instead of a DC speaker load. The capacitor could build up a high DC voltage and then it conduct and make that pop sound and then starts to charge up again.
/Ubbe