How long until a 85 300D bad bearing/rod fails or is it my Head gasket?
 

This summer I plugged my breather hose on my 85 300d and my engine ran down to 1 qt of oil. After that my engine was noisy and shook like it would jump out of the compartment, made all kinds of rata tat sounds and metal hammering sounds.
I kept the car on the road and even took it to Virgina high in the mountains, waiting for the engine to fail. It wouldnt!!!!
One day I got a brainstorm and changed the IP and suddenly the engine sounded like it was new...I found out the old Ip literally had two cylinders not running..and a badly leaking ALDA..the low oil had killed the IP..
That was 10K ago. The engine still runs.
Somedays its so quiet, it sounds new. Other days I swear I can hear a bad rod or bearing. Sometiems I think I have a hole in the Exhaust. Somedays I swear the car sounds perfect..
This morning I came in to work and it sounded Brand New! Tonight I know it will sound different. The noise is dependent on IP timing, the better I time it, the quieter it gets ( I think)

My question is , how long until a bad bearing or rod finally fails Big Time?

Could the failed IP introduce a "lean Condition" which could have trashed the Head Gasket?
I definitely hear a maetallic ring on some days after a cold start.

Could anything else be wrong?

I have really stressed this car , trying to get the engine to fail, i have taken it up mountains, had it to 100 mph...Maybe I should just get a loud Radio and ignore it...finally why do I think its an Exhaust leak somedays?

Bearing/rod
 

At what RPM do you hear the rod/bearing noises? I have read, and cannot find the page, that rod/bearing noises get much louder after 2500 rpm. The faster the rpm the louder the noises. Maybe some of the techs can chime in.

Almost forgot
 

Have you ever removed the inspection/cover plate near the torgue converter? It is oval in shape. There is a flex plate in there held in by six bolts. If a bolt is loose or missing it can sound like bearing or rod going bad. With the low oil level maybe not, but I always look for the cheapest easiest way out. Good luck!