question and opinions needed
 

I just took my truck to the shop a month ago becasue it started overheating. They replaced the thermastat, some coolant hoses, and the serpentine belt. In the last week i have started to hear the "waterfall" noise again in the dash on the passenger side. Its not currently overheating. I'm wondering what i should do. Do i need to take it back in, or just live with it until it does overheat again?

Take it back, you have a leak.
If it was low on coolant when you first took it in they should have looked for a leak before they did anything else.
So far all they have done is take your money.

Take it back right away.If you wait till it overheats it will be to late.Better safe then a warped head.

We may not always have the right answer but we always have opinions.

I agree with the others that you have a leak somewhere in the cooling system. The waterfall sound you hear is an air bubble in the heater core. Until you fix the leak and properly bleed the system you'll continue to have that sound. Take it back to the mechanic and have them find the leak.

the thermostat and belt were most likely not a bad purchase anyway, should be done regularly, although you could have done them yourself. but like everyone else said go find the coolant leak.