The first "item" got sucked into the blower compartment and stuck in the fan causing the vibration.
Seeing this the experty furnace guy deduced that if one thing made it vibrate two things should make it stop.
Before performing the delicate job of finding the perfect place to locate the balancing weight...... it was lunch time! Searching the customer's basement all that was found were cans of peas bought on sale by the customer.
After lunch and consumption of the missing PBR (look for the empty can near the peas on the shelf) a memory from training class back at the shop came to mind. If you can't fix it, use your BFH. Having dozed off during the details on how to use said BFH (another PBR break), said genius decided that the most logical thing to do was to tape it to the side of the offending object. Everyone knows that if it can't be fixed with duct tape, it aint broken!
Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.