any bump could ruin it, depend on timing and location. I can't give you an exact force measure, but I've had a laptop fall to the floor from table (about 3-4ft) and the laptop weighed about 3-4 lbs, it is fine to this day.
The noise youre hearing when picking it up is most likely a fan (if it's high pitched, if it's a low pitch sound, it could be HDD motor but I doubt it's a big problem)
what is your problem with bumping in general? did you bang some laptop and it's not booting up? or you plan to do it in the near future?
rule of thumb: treat your electronics nicely and they will live a long and good life serving your needs. if you go bumping, dropping and otherwise mistreating your electronics, well, don't be surprised if you're shopping for a new laptop within 2 years.