Welcome to the group. STick around and ask lots of questions, that is one way to learn.
The key to troubleshooting is to isolate the problem. Where is the problem, and where is it not? For example, if any controls - volume, tone, whatever - gave an effect on the hiss, then those controls are AFTER its source. And if they have no effect, then they are before its source.
So on an amp if a master volume turns the hiss up and down, but the pregain control does not, then the his is happening between those two points in the circuit.
DO you have the schematic? Fender packs one with the owners manual on most recent models.
As to what it could be, a noisy tube perhaps, a noisy semiconductor if any are in that circuit, even a noisy resistor. HArd to say. That is why we want to isolate down to a small part of the circuit.
Pulling tubes can be a method. Chances are the power tubes - more properly referred to as output tubes I suppose - are OK, they don't usually make hiss. But never assume, anything is possible. But the smaller preamp tubes can be pulled. ANy tube you pull that does not affect the hiss is before its source. Sound familiar? ANd any tube you pull that stops the hisss is either its source or at least after the source.
We always power down before pulling tubes or installing them.
Like numbered tubes can be swapped. If your preamp tubes are 12AX7/ECC83, then you can swap places between two of them to see if anything changes. If nothing chnages, the twp tubes are likely OK, but if the amp does anything different, then one of them is odd.
Isolate the problem, divide and conquer, whatever you want to call it, narrow down where the problem lies and it is so much easier to fix.
I also always like to check operating voltages to make sure none are way out of whack. No point in looking for a noisy part first if the voltages it needs for operation are half what they should be or are missing altogether.