18 DC Bubbling/ Popping noise at slow speeds

At slow speeds in both forward and reverse im hearing a bubbling/ popping noise. Almost like popcorn popping around the firewall or under the dash. Very faint and intermittent but its becoming more frequent. Ive seen a few other threads with no real ideas. Anyone got any info? Ill try to record the noise but its hard to hear.

 

Does it sound similar to sand/gravel on the wheel wells or more like a single pebble is rolling around?

 

You might want to check your coolant level if its a bubbling sound you're hearing. You might be low and hearing air in the lines being recirculated through the heater core. Maybe.
If its a wheel noise - check for the obvious rocks, nails, and other "hitchhikers" in the tread. Also - make sure you don't have any loose wheel weights. Those noises are more clicking than popping, though.
Any recent suspension work? Oil change where the skid plate was removed? Those can cause weird noises as well if the bolts weren't torqued right on reinstallation....