Very weird noise with mismatched speaker

[Fossilshark]

This is probably a dumb question but here it goes...

My friend gave me a crate 15w practice amp with a busted speaker, so i took the electronics out and ive been using it with a few cabs. First i connected it with a 12 inch 8 ohm  celestion and it worked ok. The amps output is 4 ohm so to get closer i took two practice amps i turned into speaker cabs and wired them up in parallel so i had 3x8 ohm speakers wired in parallel. This worked great for a while but recently theres been alot of loud crackle. Like when you plug in an instrument but way louder and harsher. Im running just one 8 ohm now and every time the guitar makes a noise that loud crackling happens. I tested it with my bc rich with humbuckers and thats when it makes the noise the most, my strat doesnt seem to trigger the noise at all. My 8 ohm celestion cab is for the tube amp im building anyways so i was just wondering what the problem is here.

[Fossilshark]

Update: happens with both guitars

[phatt]

The amplifiers output has a minimum load of 4 Ohms. Go below that and expect trouble.
Two 8 Ohm speakers (in parallel) is as low as you can go without causing damage.

At 4 Ohms it will be a bit louder but this puts TWICE the current demand on the amplifier and as it's only 15 watts low frequency notes will splatter a LOT more (and earlier on the volume dial) and you may have already damaged the chip inside by driving less than 4 Ohms.
Phil.

[Enzo]

Does it just do this with THAT one speaker?  Or does it do this with ANY ONE speaker?

And connect one speaker to it, and turn it on, and no guitar, but ball up your fist and whack the top of the amp, does it make the noise when whacked?

[Fossilshark]

i would love to use the mis-engineering compensation tool (fist) but there is no chassis on top and its already bit me a few times for poking too close to the power transformer (10/10 ESA recommends). it does this with all speakers. i may have damaged the chip, thanks for the input.

[Enzo]

Whacking is a valid technique, I was not joking.   If your fist isn;t handy, use a rubber mallet. 

I was assuming the little chassis was bolted into the little cab, but you can whack the end of the metal chassis, hell use a heavy screwdriver handle, or really even a metal hammer.  We want to send a mechanical shock through the thing, that will expose loose/cracked connections or even parts with internal issues.  If we find anything is sensitive to being hit, we then know to look closer for the individual thing causing it.

Larger parts on the chassis sometimes vibrate enough to crack their solder, then the vibration from sound can cause momentary loss of connection.   Larger parts are the main filter caps, larger power resistors, and also things along the panel like jacks and controls.  They can crack their solder.

Poking on the board is best done with an insulated probe, which is a fancy way of saying a wooden chopstick, like from Chinese restaurant.