2003 Infiniti I35 • 6 cylinder Automatic • 180,000 miles

I have a 2003 Infiniti I35 with 180,000 miles. A few days ago I felt and heard some vibration coming from the engine. I turned the heater off to listen for it and the vibration stopped. Turned it on, vibration started again. Aha! I thought. Blower motor. I removed the blower motor and hooked it up and turned it on, holding it by the base. I was surprised that it ran very smoothly. I disconnected it and left the blower motor out until I could pick up a replacement. Out of curiosity tonight, I turned the heat on (with the blower motor still detached) to see if I might get some heat through the vents while I drove. I didn't heat, but the vibration started again. I opened the hood and noticed the radiator was vibrating a lot when the heater (sans blower motor) was on, but not at all when off. Any ideas?
January 15, 2011.

That is an interesting one. Make sure there is no air block in the radiator or heater core. Make sure the engine isn't overheating.