Engine and trans mounts are probably tired?
Check the ground strap for condition.
A bit of firewall insulation, and below the floor can reduce noise and improve cab temps.
Check your recirculate damper/linkage and pivot stud for cracking and condition.
Add insulation to your AC ducts if its missing
Replace oil cooler gasket.
confirm Aux water pump is in good shape.
Consider replacing your OM612 intake manifold with the later OM647 version. I am not sure, but the new EGR valve design may be compatible with the ECM/harness, and it is MUCH easier to work on. You will need to drill and tap 1-2 holes in the head, but its pretty straightforward.
Consider an A/C system refresh. New dryer, O-rings, TX valve, and recharge. drain some oil from the compressor housing drain bolt, and check for condition. If its a nice clear amber, replace the oil removed, and it should be good for a long while. Check the compressor clutch air gap as well.
Flush your evaporator core out. Spray with foam cleaner and then rinse thoroughly (in place).