As the temperature started dropping with the change from summer to autumn over the past several weeks, I started hearing water "rushing" sounds behind the glovebox every time I started the engine, but only if the engine was cold.

After letting the engine cool down, I would open the radiator cap and find that it was a bit low on coolant. At first I just tried topping it off and burping air out of the system, but the problem would just come back after the next full warm-up/cool-down cycle.

I couldn't find any signs of leaking. Scary possibilities started running through my mind, like a slightly failed head gasket either allowing coolant to be burned, or allowing combustion gasses into the coolant.

The total amount of coolant in the system (including the reservoir) seemed to be stable, so that ruled out leaks and burning of coolant. But somehow air gets into the system. I finally decided to just go through and re-tighten all the heater hose clamps (all connections on the new heater hoses that are part of the turbo kit).

The water rushing sound diminished with each warm-up/cool-down cycle after this, and was completely gone after about 2-3 days! It's now been several days with no problems. I'm guessing that a combination of cooler weather causing more extreme temperature swings in the rubber hoses, along with the hose developing a "memory" of the clamp compression, caused the connections to loosen up just enough that air could be sucked in past those connections as the coolant cooled down and contracted, but not loose enough to allow coolant to leak out.

I wonder if it might have been better to use re-use the original spring clamps, which will maintain a steady clamping force even as the hose compresses and develops a "memory". At this point, it's not worth the effort and mess to pull all those hose connections apart and swap clamps. I just hope I don't ever have to re-tighten them again.