A number of issues.
1) Bad checksum probably means the battery backed ram is on its way out. Look at "Fixing internal memory problems" in the UM. If it comes back after this, you need to replace the ram.
2) Your feedback problems are most likely a feature of the way the unit is connected – it won't do it by itself.
3) Your hum is probably a ground loop, which cannot be fixed remotely. First step is to plug the 4500 into the same outlet as your computer.
(2) and (3) may need local technical help.