Bubbling sound from left passenger seat

I have a 2006 Prius with 60,000 miles on it and about 1000 miles ago I started hearing an intermittent bubbling sound coming from the left rear seat area. It almost sounds like the car is percolating coffee (but no coffee smell or any other smell). It occurs only when the car is in motion, but it seems to have no association with road bumps or going faster or slower. It has no association with how hard the throttle is deeply the throttle is depressed.

I did a search and read through some of the comments regarding people hearing similar sounds, though most described a sloshing which is not what I hear, I hear a bubbling type sound. Most commonly it was suggested that there was water trapped somewhere in the trunk area (not entirely sure where) from rain storms. But I live in the desert, and I park in a garage, and my car has been rained on all of twice in the last few months (I've been wanting what we call an Arizona car wash which is a lot of rain that washes the dirt and dust off your car but have not gotten one). While we do get the occassional rainstorm there are lots of times when we are so dry water molecules never get a chance to consider condensing much less staying for a while. The bubbling sound was present in that driest season when our dewpoints were below zero. And finally I have gone to a car wash once in all the time we have owned this Prius (and never again because they didn't get it clean and let a smoker drive it) and that was at least 6 months ago.

The place where I've been taking my Prius to be regularly serviced (not a dealership but has a certified Prius mechanic) has mentioned that they would like to replace the inverter coolant pump at around 60,000 miles. They also mentioned that they like to change the coolant for the battery and/or the inverter (can't remember if it was one or both, I was in a hurry and didn't write it down) at around the same time. Estimated cost for these services somewhere around $650-$700. The car has been very reliable and has needed only oil changes the whole time, but I'd like it to last another 60,000 and then some, and I don't want to get stuck, especially at 108 degrees on the highway.

Is it possible that this bubbling sound is a sign of impending trouble from the inverter coolant pump?

Is the service station just trying to make a buck on an unneeded repair or is the inverter coolant pump a legitimate concern?

Could this bubbling sound be due to any other problems with either the hybrid battery bank or the inverter?

Is this a completely benign sound?

 

Thanks, to both of you, will give all your suggestions a try when I get a chance (may take a few days). Tires is interesting thought, I did replace the original Integrities with new Michelins about 4000 miles ago, hmm yes that would fall into the right time frame. The sound does not appear to vary with terrain. I could have the service station rotate the tires and see if the sound moves ... Traction battery fan, interesting idea, I live on a dirt road I'm wondering if maybe somehow a pebble or two could have gotten in there and be bouncing around when the fan turns, that might make an irregular bubbling sound. However I don't hear it at stop lights at all, which I would think would eliminate the fan so long as it also turns when the vehicle is stopped. But this might be worth looking at just in case. No problems with the running of the car, no indicator lights, and I'm getting 54mpg.

Anyway thanks again, some good thought and I'm relieved to know that it is unlikely to be the inverter coolant pump, also for the timeframe on replacing that.