I bought a nice looking Cobra 25 at a garage sale. It powers up nicely, when I connect the mic I get a rapid and loud thud thud thud thud coming over the speaker. I thought it was the mic so I tried 4 other Cobra mics and same thing, as soon as a mic is connected the noise starts. Hooked it up to 2 other power sources and still the same thing. Anyone ever encounter this? Any thoughts where to start?