I get that rushing fluid sound in the heater core and coolant lines all the time when I am using the heat. I've heard it in every toyota i've ever driven. It's not a HG problem for me.

I would reccomend that you not worry about it too much and keep a close eye on the coolant and oil (smell the oil as well).
If you are truly concerned get a leakdown test performed.

Theresa the Toyota Terror-89 Toyota P/U (3.0L SR5, 5sp swapped in from auto, 3"B.L., 32X11.5X15 MT/R, 2.5" stainless magnaflow catback exhaust, leaky floor with cold draft @feet, FUBAR comptron now in glovebox)