Need help fixing a HP Laserjet 4P printer. Makes

it could be a bad toner cartridge
try replacing each toner cartridge one by one to see if grinding gear noise goes away.
Or
it could be jammed paper somewhere inside the printer

Hi there,
The noise your hearing is caused by your heating unit. We see this problem regularly.The Heating film is busy ripping/tearing and it will cause that noise and give jam errors coz there is a sensor inside the heating unit. You will need to take the printer in to a service center so that they can replace the heating Film. Shouldnt take to long to get fixed by a service agent.

regards,

The grinding noise is caused by a cracked motor gear. This is a very common issue on the Laserjet 4L and 4P units. It is a fairly easy repair but you will need to disassemble the printer to get to the drive motor. The gear is available here:

http://www.ccl-la.com/catalog/product_info.php?cPath=24_41_49&products_id=61

Common issue with these older 4L printers. The problem is a small plastic gear on the main drive motor has cracked. This is what causes the noise and since the machine is not "running" there is no paper feed and thus the paper jam light. Basically there is no drive to any of the printers components. May be hard to find a replacement motor these days but that is what you would need to fix the printer.
wheatlandsvcs

http://h10025.www1.hp.com/ewfrf/wc/document?docname=bpu06191&cc=us&dlc=en&lc=en&jumpid=reg_R1002_USEN

pull your fuser out and you will find the jam and reason for the noise. it is pulled from the rear 2 covers and 2 screws

Most likely a bad pick up roller these machines will give a jam error if the paper or envelope is not picked up. Try cleaning the roll for the tray you are having the trouble with if the condition improves you have found the problem.
Hope this helps

CHECK RS PRINTER PARTS FOR HP LASERJET 2400 2420 2430 PRINTER GEAR KIT

Hi,

A 50 Service error is a fuser error. Turn the printer off for 5 to 10 minutes, then turn it back on. If the error is still there, I think your going to need to make a service call you can try reseating the fuser, but be sure to let the fuser cool completely before touching it.
Good Luck the HP Laserjet 4p is a great printer.