I tore apart the engine in my old '70 911T for a rebuild and specifically to cure a rattling/grinding noise coming from inside the crankcase. Bought all the parts, had all the machine work done and was closing up the case when it occurred to me that I didn't check the driven gear on the intermediate shaft for wear. Having already invested several weekends and having dropped a good deal of money on parts (as well as being being anxious to get the car back on the road) I just said
"Screw it" and left the old shaft in there.
After getting the motor all back together, re-installing it in the car and hooking everything up, I hit the key and fired it up. Much to my annoyance, that friggin' noise in the crankcase was still there! You know the rest of the story...it was that worn gear on the intermediate shaft making all the noise...the one moving part that I didn't either rebuild or replace
Trying to save some time and a few bucks caused me to tear the motor completely apart for a second time to install a new shaft. All quiet after that and a valuable lesson learned.