Bubbles accumulate in the top of U tube overflows due to insufficient flow.
Since you talked about having less flow, I'd say you've found your own cause. Slurping in a drain line usually is caused by a belly in the line, or an airflow problem where the line is not allowed to "breathe" properly. Since it came up along with your flow problems, fix that, and see if it goes away.
You've not cleaned out your pump in 4 years? You can replace it if you want (Rio isn't that great anyway), but you might try dismantling it and giving it a complete cleaning. You can soak the pump parts in either a decently concentrated mixture of CLR or a well diluted mix of muriatic acid, to get all that crud off of the parts. This is the easiest way, that stuff can be nearly impossible to remove by hand scrubbing, plus hand scrubbing tends to break delicate impellers, etc.
I actually prefer to use muriatic acid. Although it's more dangerous to work with, it's actually safer for the tank. Should trace amounts get into the tank, it's not going to hurt anything, but I have no idea what's in CLR that could cause harm. Once done soaking in muriatic acid, simply hold the parts under a running sink, then put them into a container of water and baking soda for a few minutes, which will neutralize any traces of acid.
