I've never heard rustle used that way. In fact, I'm not sure I've heard any term used for that. I would say, if I had to: My pant/trouser legs are rubbing against each other. And since that's so awkward, I wouldn't say it at all.
As Copyright has shown in post #2 there is an AE/BE difference here. If you say 'pants' in the UK most people think you are talking about underpants, and those of a woman as well. Which could be unfortunate.
I would never say "squeaking" about any article of clothing unless it's the sound of rubber against a wood floor or something.
Rustling = the sound of dry leaves moving against each other. But yes, sometimes people use it for clothes, mainly loose clothes. I have to agree with copyright, it sounds so awkward trying to come up with a term to describe the sound of pant legs against each other. I'd just avoid it, or if I had to comment on it, I guess anything would work, but nothing would sound right.