I have paint cans in the house. I noticed a popping sound coming from one that has been used and resealed. Why are they doing that? And is it dangerous, safe, normal?
Is the temperature changing relative to what it was when they were sealed? The air inside the can could be expanding (with a rise in ambient temperature) or contracting (with a drop in ambient temperature), which could cause the can to make noises. Removing the lid and re-sealing it should relieve the pressure difference.
Alternatively, you may have sealed a small animal inside … have you checked for that? 
Yes, the temperature has changed since they have been sealed. I will try and reseal them and see if that helps.
I am now opening the can for the reseal…OMG!!! It’s Foo-Foo the fluffy hamster. So, that’s where you’ve been hiding the past month ;).
Poor Foo-Foo!
Probably not so fluffy any more, having been dumped into a half-empty can of paint…
It could also be differences in barometric pressure.
I just got mine from the garage and I noticed a popping sound like crackling and some animal like sounds I’m not opening it 
Barometric pressure doesn’t vary much in any one location, maybe half a PSI from min to max. A 1-gallon paint can is maybe 7" across, making a lid with 38 square inches, so that equates to 19 pounds of force. This assumes that you sealed the can at a time when barometric pressure was at its lowest.
If the ambient temp increases from 70F to 87F, this would also increase the pressure by about half a PSI.
Here’s another theory:
When you open the can, you let ambient air into it. This air is devoid of whatever solvent/carrier the paint is made of. Then you seal the can, locking in a pressure that matches ambient.
Now the solvent/carrier in the paint evaporates until the partial pressure of the vapor equals its saturation pressure. This results in a total pressure inside the can that is greater than atmospheric. If the lid is poorly secured and/or large (large lids have more surface area, therefore greater force, per length of perimeter), the increased internal pressure may be enough to pop the lid loose or force a leak.
Once the pressure equilibrates, the vapor pressure inside the can will be lower, but only to a limited degree. You seal the can again, and now since there’s already some solvent vapor present, the new increase in pressure doesn’t have to be as great in order to achieve saturation pressure.
For water (relevant to water-based house paints), the vapor saturation pressure (at room temp) is about half a PSI, so if you’ve opened/sealed the cans in a very dry environment, this could provide an effect comparable to the above two. If you’re dealing with automotive paints, the saturation vapor pressure is much higher, about 2 psi, so you can produce a lot more force to pop a big lid off of a can.
If my theory is correct, then you could perhaps alleviate the incidence of lid-popping by gently setting the lid in place for a few minutes before finally hammering it home. Setting the lid in place without sealing it for a few minutes allows you to corral an air mass that can then become saturated with the paint’s solvent/carrier while maintaining equilibrium with ambient pressure. Once it’s saturated, you can seal the lid tight, after which the can will only be affected by changes in temperature or ambient pressure. (the risk of this method is that after you set the lids in place, you might forget to return later to fully seal them…)
Note that the actual “pop” is because (1) The top is sucked in (or out) by air pressure or temperature change, (2), the top “domes”, bending and expanding to make a dome, (in or out), (3) the stretched metal can’t go back, won’t fit, doesn’t “shrink” like it expanded.
So (4), when the temperature/pressure changes enough, eventually the dome snaps through, from one side to the other.
“Crinkling” is mostly the side of the can. As the air temperature/pressure changes, the sides of the can crinkle in or out. Unlike a “pop”, which only happens once, but is loud enough to notice, crinkling keeps on happening, but you only notice it when you are close enough.