It could be arching effect, Happens when spinning the plug around, So the contact between the to metal plates inside the plug probably makes a humming. It's common on all, I got their 400 Lead Acid and it does same thing. So it's not a defect of the product. Though there is 1 thing i noticed, And not sure if it's just my 150 unit, or it's all. (So you can test this if you do get one if you wish.). Charging VIA 12volt (Ether car/ or external solar panel rated at 15volts or less. vs 18volts or less.). That it'll just hang at 50% charge when it's plugged in to charge from a 12volt plug. (Doesn't happen when using THEIR 18volt 12volt solar panels. But i have bruntons explorers with 40watts (2x 20watts), full sun and left few days.) just doesn't go over 50% (it'll blink at 60%.) But if you use the wall charger or their solar panels to charge, It charges normal. But normal 12volt with a 15volt initial power, won't cut it.

Sure will test that when I get my 150. Currently trying to source the 9mm chaining cable for my pair of sherpa’s. Rare parts....