I love collapsible stocks on ARs, but I hate how they rattle.
Do any of you have any easy fixes for that?

This is one area of the AR where Mil-Spec or Commercial can make a difference.
Commercial has no standard so chances of rattling are greater with Commercial.
If you have a milspec buffer tube with a Commercial buttstock it will be a rattler.
You can you use tape to shim or make sure you have a good milspec tube and buttstock.

^^^^^ This^^^^^

Several aftermarket stock manufacturers provide shims so that their stocks fit either configuration. But you don't want to put a commercial stock on a mil-spec tube.

RSS. Hint………..

Aluminum foil tape. I have some made by "Reflectix" for taping together their insulation. It is simply a thin strip of aluminum metal with sticky goo on the back side. It can be very carefully applied to the buffer tube to build it up and tighten up the stock.

Some stocks are easier to do than others. Some are not exactly straight and even inside the channel from front to back, and those take some more creative taping to get them to do. And it will probably take a lot of peeling off and retaping to get it all right, but once it's done, it's done.

Even the cheapest stock can be reshaped to fit tight on the tube with a heat gun.

But I buy SOPMOD’s or CTR’s.

CTR stock.

Pick a LOP you want and pack the thing full of JB Weld. Rattle gone.

Or just keep it simple, and buy a bolt action. No rattle

MOE?

Such a super solution. Buy an adjustable stock and then make it fixed. Or come to an AR forum and suggest toss it and buy a bolt rifle...
You must be a Mensa candidate.

Kenmi is a couple cards short of a full deck.

If it’s loose as, spray paint the buffer tube, then run a tape across the spine.

Or buy a stock that locks down like a ctr

CTR’s lock down, MOE’s don’t.

accidentally ended up with a couple that lock. Had no clue that was a thing. Then thought reordered same and they didn't come with the locks. I like the locks. Pretty cool thing.

I love the locking ones, like the moe, and gave away all my basic m4 style stocks