I have a second hand fender twin amp and when you turn it on there is a lot of crackling and noise. Its been getting worse over the last month or so it used to die off after about 5 mins now its just constant. Iv tried tapping the tubes and cant hear any noise and iv also tried swapping them around a bit but still nothing.

Any ideas whats up?
It's next to impossible to trouble-shoot amp problems over the internet (yet that doesn't stop people from trying).

The best advice I can give you is to take it to an amp tech that you can trust. I wish I had a magical fix for you but unfortunately, you're probably going to have to spend some money to get it repaired (like everything else in life).

That said, I've quite a bit of experience with different amps over the years. I've found fender amps to be some of the noisiest amps I've ever used and they are extremely sensitive to picking up and amplifying noise. So, some of this may just be the character of the amp (How many pedals are you plugging into it?). Although, if it's highly noticeable with zero pedals plugged in; Then you'll definitely want to have it looked at.
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nah theres nothing plugged in and its a loud crackle regardless of what volume.
it was just for a bit after i turned it on but now it seems to be constant, its unplayable so i havent really tried to see how long it stays for.