Define "strange"; it could have been a normal hard drive noise that you just don't notice usually, it could have been something vibrating due to fan or hard drive that is dampened when the computer is on the table, it could have been the optical drive doing its power-on "whirr-click" thing, if there was an optical disc inserted you might have actually unbalanced it by moving it, causing it to make an unusual sound, or it could have been somehow related to the speakers, maybe a power grounding issue that was exposed when you held it out of contact with the table. Or it could have just been your imagination.
Point being, you're going to need to be more specific than "strange buzz" about exactly when during the startup procedure you heard it and exactly what it sounded like to get a useful answer.
Try rebooting now, and don't lift it up. Same sound? If not, try rebooting again, and lift it up at the same time you did before. Any noise?
