I have insulation and trash coming up through a couple of my vents, nothing loose under house...?

Rats, racoons, or some other vermin are doing a number on your floor insulation.

Papaw

The insulation inside your duct work is red. Odds are that a rat or a squirrel has taken shelter inside your duct work , and is chewing up the insulation inside the duct work and when the air comes on it's blowing the loose insulation up to the vents.. I'd do a better inspection under the house. More than likely it's a little hole that they're going in and out of , I'd recommend setting some mouse traps while you're under there too.

I think FOXES is on the right track. I have seen duct liner come loose with age so it could be just that or the animal thing. I would borrow or rent a see snake and look inside the vents if you can get the registers loose. You also may be able to get the see snake into the duct below by pulling one of the boots loose under the house. If a big section of duct liner is loose it could be blocking your air flow which can cause multiple issues.

You might check for pests such as ants, mice, etc.The other possibility is a space between the vent and the duct. You need to have a tight seal between the duct and the vent connection, this is not the vent cover as most of the have a seal on the contact edge