Back in the day I worked in a TV shop, we used a "green stick", actually a fiberglass rod as a n insulated stethoscope to pinpoint noises in the HV cage. Garage mechanics used metal rods to pinpoint noisy lifters and such in engines. Since active tuners have high RF voltages present and are detuned by metal where it's not supposed to be I would use a fiberglass rod with one end pressed against the bone just in front of the ear canal, fiberglass and bone conduct sound much better than air. For that reason I would have someone else speak into the mic, or better yet use a test tone, or best yet give your transmitter a two tone test, a two birds with one stone approach. Do I have to tell you a dummy load is better than crapping up the band with on air tests? They are the court of last resort, if your antenna presented a 50 ohm load you wouldn't need a tuner... read on.

One last thing, I have a bad feeling about this, the last time I heard a tuner "singing" it was internal arcing, the internal balun screaming in pain. This was surprising at first, an MFJ "3KW" tuner arcing with only 100W of AM, modulation struck the arc followed by the hiss of carrier arcing inside the balun. It was more the antenna system than the tuner, a 160M sloping open delta fed with open wire ladder line. It turned out the line was a half wave on the 75M AM Gangsta band sending the high RF voltage at the antenna feed point down to the tuner, baluns like current feed, voltage feed kills them. This is where the old Johnson Matchbox shines over today's tuners, it has a balanced output and one leg can be shorted to ground for unbalanced output, it laughs at high voltage.