Engine Cooling problem
1991 Toyota Pickup 6 cyl Four Wheel Drive Automatic 220,00 miles

I am experiencing a strange cooling issue that started last week. When I shut my truck off I immediately start hearing a loud bubbling noise from under the hood. I have discovered that it's steam coming out of the radiator overflow tube and bubbling in the reservoir tank. I thought it might be a bad rad cap - so I replaced it - no change. The temp gauge does not register hot (ohm'd gauge just to be sure and it's checked out OK). When I check the level in the morning coolant is slightly low - maybe a 10oz low. I have a relief valve on the rad cap, and when lifted a rush of steam comes out the overflow tube into the reservoir. It's really strange. It may be happening while driving however I can not hear it over the engine noise. My truck is my pride and joy and well taken care of. All maintenance is up to date. I did the timing belt/water pump at 208K, and used OE parts. Any ideas - I'm stumped. Thank you in advance :)