where did you find this funnel? i have not heard of nor seen a funnel specifically for cooling systems. do you have a link for the funnel ? if it took care of your sytem problem i'll definitely give it a try.

 

I also have a 2001 dodge 5.9, but mine is 4/2.
I had the same exact problem hearing coolant flowing through the heater core.
I have been having overheating problems and have changed the thermostat, radiator hoses, fluid and cap.
No more coolant noises but massive over heating.
Now it's time for a new radiator because after over heating to many times I have 3 visible leaks.
Good luck with your's
doorman 2

 

The funnels can be found over at summit racing for 20 bucks here's the link
http://www.summitracing.com/parts/lil-24610
Also I've heard that the reason these rams have this issue is that the heater core sits a couple inches higher than the radiator cap. You might try lowering your heater core and see if that solves the problem. I had a 97 ram with the 360 and after replacing the radiator, water pump, heads, heater core three times, thermostat, and having the guys at the local dodge dealer say they didn't know what was wrong I just sold it. But I always wondered if lowering the heater core would have solved the problem. Now I'm looking at a 98 ram and if it has the same issue I'm going to lower the heater core and see if it works.