I thought the trapped air in the heater core (audible 'slushing' sound) will eventually be flushed out on its own.

I have read squeezing the tubes going through the firewall helps.

Is there other method (like taking the coolant cap off once in a while?) that helps?

Or will it go away on its own?

 

Jack the front of the car up. So that the air can only flow up hill, and run the car through a heat cycle with the rad cap 1/2 on.
Do it with the car cold and the heat cycle will push air pocket out or at least should. Rad cap 1/2 off/on keeps the system from pressurizing so air pocket can move.
Air pockets suck hope this helps....

 

Found this one elsewhere. Does this apply to the B5 A4 as well?

Pull the corrugated plastic cover off the two heater hoses where they pass into the inner firewall.

Loosen or remove the passenger side hose clamp and pull the hose off the flange a short distance. This uncovers a bleed hole in the hose so the air can be pushed out by cooling system pressure.

With the engine hot and idling (expansion tank cap on and sealed) the system will pressurize and spit air out of the hole.

When all the air is gone you'll get a steady stream of coolant coming out of the hole.

Slide the hose back on the flange and tighten the clamp.

 

Interesting. I've been hearing that slushing sound since I replaced my lower radiator hose in the spring, and noticed now that it's getting colder that I no longer have heat. Sounds like this might be something I should investigate!

 

Yes, there is a bleed hole there where hoses go into firewall, I have done this to bleed before.....