How many miles?
I don't think the rails are the source of your noise, but I could be wrong. You more than likely need a timing chain and tensioner if over 100K and never been done. If you're over 350-400K you might need rails (upper and lower). The older cars have aluminum rails which don't wear out as fast as the plastic rails.
Might also need a valve adjustment while you're in there. Should be adjusting them every 15K or so on the 4.5 (same interval as the diesels).
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.