I purchased several new LG MJ1s, but when I put them in my Convoy S2+s (5A driver), they emit a high pitch whining sound. There is no sound if I use Sanyo GAs. Is there a problem with these batteries?
I don't think batteries emit whining sounds.
I'm thinking it's your flashlight that is emitting the whining sound, though I don't know why.
It’s probably from the flashlight, but only with MJ1s. It is loudest at the 35% and 100% modes.
Check that the brass pill is tight and also the driver retaining ring.
The sound seems to come from the tail. I tightened everything and the sound is still the same. Don’t know why the battery makes a difference. I tried 2 S2+s and it’s the same situation with both.
Did you also tighten the switch retaining ring? I believe it’s a reverse thread.
The high frequency PWM signal in combination with high current can cause audible noise. Parts in the driver or battery will work like little speakers with the changing magnetic fields (this reminds me that I still want to test some flashlights for EMC at work). When batteries are new they might still have small gaps between parts that might close after some time. Sometimes it’s just the way the battery was built.
I used to get a whining sound like that… Turned out it was my mother-in-law 
Yes I also tightened the switch retaining ring. I tried the MJ1 in a Convoy M2 with 5A driver (I believe it’s the same as the S2+ driver), and there’s no sound. There’s also no sound with the MJ1 in other lights. So it’s something specific to the combination of the S2+ (probably the switch) and the MJ1.