MK136 Super Stereo Ear Problem

I’m new to electronics, but the stereo ear is the 4th small kit I’ve made. I´ve finished it, & all the joints look OK through a small magnifier, but I’m getting a loud mid-tone hum which drowns out virtually everything.

The odd thing is that if I either -
a) hold the unit on the palm of one hand and hold the other hand flat and as close above the unit as possible;
b) rest the unit on a surface and put one hand each side of it, palms inward & hands parallel & an exact distance which is about 2 inches away from the unit;
then the hum disappears, and the unit amplifies with perfect clarity- like it says on the box.

If I move my hands an inch or so from the sweet spot the loud hum is back at once. It doesn’t seem to make any difference where inside or outside the house I am & I’m not aware of anything odd in the environment. I googled & also looked at all the posts in this forum and found no mention of a problem like this.

Any ideas on what I might have done wrong?

Make sure to power the unit from batteries, as intended.
Check the connections of both microphones.

You have solved my problem. I checked against a Wikipedia sample file, by the way, & the noise was 50Hz - & very loud. It turned out to be the ground connection on one of the mikes, as you said. They looked good as far as I could easily see from the outside of the connections but eventually I peered into the space between the mike & the solder joint & I could see that on one mike the joint on the flat prong looked dodgy. A lump of solder worked in round the back has sorted it - no noise at all.
Thanks a lot!