2001 Freightliner MT45 issue please help!

Post the last six of VIN for more accurate information.
Here's a troubleshooting guide that may give you so direction.
Schematic for the Delco Remy but the troubleshooting is for the Nippondenso

 

That is the starter solenoid chattering. Sounds like a there's a very poor connection somewhere in the starter circuit. Time to break out the multimeter and start checking the voltage drop on the power and ground side of the starter circuit.

 

I checked the last six of your vin and shows a build date of 4/10/2000 but wiring info specific to VIN is only available for vehicles built after January 1, 2001. However, did see that your starter was originally a Delco Remy. The Nippondenso troubleshooting guide could still help you to do testing. You never mentioned anything about the starter relay that per manual is mounted on engine side of operator station, be sure and check that. The voltage drop testing on ground side and positive side should help narrow the problem down.