Thank you. It does have the Effects Bypass switch, but it seems to crackle regardless of switch position. I will check it some more, but I'm pretty sure. The crackle sounds like a dirty pot, except the pots are holding still in the off position.
Failing tube?
The fan is a very easy replace. Just minor soldering required, or you could simply cut & strip the wires and twist & tape them together with the new fan if someone doesn't want to, or can't solder.
I don't recall what the airflow requirement is. AgedHorse or someone else will know. There are many more high-quality silent fan brands now than back when it became fashionable to replace them.
Just be aware that it isn't the fan itself that's noisy, it's the airflow through the slats and the casing vibrating. Try running the fan when the case is open, or more so when it's detached from the case and you'll see it's silent. If you're very handy, you'd likely get much more silence by removing some or all of the slats than by changing the fan.
IF changing the fan, look for one that comes with rubber bushings (or make your own) to dampen the fan itself vibrating the casing.
Another good trick is to try to loosen or tighten all the knobs and screws, or put pressure on the case in various places, to see if it gets quieter.
I have a few amps by another brand that are identical, and one has a "fan" that's much quieter than the other two. "Fan" in quotes because all three fans run totally silent when not attached to the cases.
By randomly adjusting various screws on the case, one became quieter and the 3rd became less noisy by bending the top panel a bit. I got the idea when I noticed there was no bothersome sound if I simply placed a hand on it. Nothing was actually loose and vibrating, just the case itself was more resonant. With so many screws, panels, knobs, etc. that's no real surprise.
It can drive you insane chasing faint vibrations in amps, cabs, cars, etc. It's usually not what you thought it would be.
As for your whole combo vibrating and making noise, it's possible the amp isn't snug in its compartment. There are foam strips on back of the removable panel which deteriorate over time that are meant to keep the amp snug. Could be it's rattling around slightly in the compartment. Try some other form of padding to see if that helps.