Shower floor makes noise

I have an old fiberglass (?) shower, apparently made up of a floor and three side panels. When you step on one spot, it makes a tortured sound; like two things rubbing together under pressure.
I have had the house for 5 years, and just noticed it last year. It seems to be getting worse.
Any idea what could be causing this? Is it serious, or just annoying? How do I fix it? I have some fiberglass and epoxy left over from a boat project; would it make any sense to reinforce the top surface of the floor? (presumably the real fix is from underneath, and that would have to be much more work than a simple reinforcement)
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Two things rubbing together under pressure.

My guess is that the drain line goes under the area in question, and the floor joist was overdrilled for it and has now cracked. If that's the case, yeah, you'd want to fix it, and no, I don't think any surface fix would help much.
It could be less serious, e.g. just some junk caught underneath the shower base or a flooring nail which missed and is rubbing against a joist. In that case, just don't step in that spot....
--
Y.

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I have been in the crawlspace under the shower, and the joists are okay. The trap is seriously rusted, but that wouldn't cause the noise. (not looking forward to replacing it, in the room available)

I tried that, but its about a quarter of the floor.
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but on the other hand it could be rott, caused by trapped water, in time you will have to pull it up to see the damage, Plan on it .....
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