I bled my cooling system twice yesterday, the first time on a hill using the bleed screw. As soon as I drove back over the top I could hear the air in the heater core. So later that afternoon I tried the not-too-highly recommended lifted and open reservoir method while the truck was running. That got 99% of the air out, I could barely make out the noise so I thought I was on the right track.

Well this morning I take a drive on the interstate (40 miles @ 65mph), park the truck for a mountain bike ride, and then when I start it back to head into work the noise is back. It has to have gotten worse on the drive in the morning, but that doesn't make sense to me. I either left a good bit of air in the system and didn't notice it driving around town or is it possible it's getting air sucked into the cooling system? Given the positive pressure of the cooling system that doesn't make sense to me.

 

the proper way would be to bleed it cold threw the bleeder screw in the upper hose,
but you have found several other that have been tried.
the only way i see air getting sucked into the system would be the return hose to the water pump, every other part is on the pressure side of the pump.
I would be doing a C02 test on the coolant looking for exhaust gases

 

pressure test for leaks. a slight leak may not allow air introduction, while being pressurized, but can suck air while engine is off and system is cooling down. endless cycle (I've been there). or as mentioned above, source could be internal

 

The manual does say to raise the reservoir during filling/ bleeding procedure. Have you noticed if the level in the header tank is remaining constant or is it getting low each time?

When I had my D2 it always had that sound in the heater and I could never get rid of it, but the level in the tank never went lower than the min level. If I filled it it would always drop back to min, but never over heated. I think it needed a new pressure lid but I never bothered about it.

If you are losing more coolant than that have the cooling system pressure checked.

 

I haven't noticed any leaks and the level seems to be holding steady at the full line. I'll test it again, probably try the UV dye as well. Might as well make a party out of it!

I'll also check out the co2/block tester. Looks like its only $25 for the tool and $9 for the fluid from AutoZone.

 

In the mean time following the correct bleed procedure would be your best course of action. Then you are at least starting from a known place.

As DR Password said above, elevate the reservoir and bleed from the screw while the engine is cold and not running.

 

I have bled the system using that method already, I've even done it with the front end of the vehicle raised. Bled until I had a steady stream and tightened the screw while it was still flowing. Started the truck up and the waterfall is still there. I finally had enough and tried the "other" method. Just being honest I had better results with the "other" method.

 

You may just have to live with it, I know I had to in the end.

 

there is a reason, it is not something to live with

 

Whatever the reason is, its the reason proper bleed procedures are not working.