It sounds like the battery connection is not doing a good job. When a battery terminal makes and breaks contact you'd expect loud pops. I have had that with a couple of basses.
The problem is that those 'easy load' battery compartments rely on pressure to make a good contact between the battery terminals an the contacts in the battery holder.
Some batteries are slightly thinner than others, which can contribute to the issue (I stuck to Duracell because they seemed consistently good to me, but this was not a systematic test, just an impression I got), but the main issue is those contacts that are just a bit of bent metal. They're bent so that they press against the battery terminals, but over time they can lose their 'springiness' and the contact can be erratic. You can bend back the contacts so that they press against the battery again and that will fix the issue... for a while. In my experience, once that happens it will continue to happen. You can bend the contacts back, but after a few months you'll be in the same place. At least it only takes a couple of minutes to do. It may be easier if you unscrew the battery holder first.
If you want a longer term fix, then replace the battery holder. You can get an identical one for around £10, solder a couple of spots, screw it in place, you're done.
If you want a better fix, find a battery holder that fits in the existing hole which use the connectors that clamp onto the terminals.
