Just thinking out loud here. Sounds to me like that power plant may only be running on 2 or maybe 3. Way to much shaking going on.
Never mind that you had only a few bubbles, there should not be any of those,,, after all, it's not the bath for the missus is it?
If I am right on it not running on all four, then why isn't it? Could be that 1 cylinder is out and not combusting, but also leaking into the coolant. If it's not combusting, there wouldn't be any combusting gases to test for. Maybe pull the plugs and see what they look like, and do a compression or leakdown test.