I have a problem and don't know where to start. It first started with a rattle/rumble that seemed to come from the 203. It got louder and increased in speed with the truck's speed when I was on the gas. No noise at same speed when I let off the gas. I babied it for about a week, but got on the gas hard a few times the other day. Now I have the same noise from the 203, and also a clunk from the rear end area with almost every tire revolution. Also, I get clunking/rattling when I coast now. Some information that may help. 1976 Blazer didn't run when I got it, so I don't know about any noises. Installed rebuilt 350 and new TH350. Drove 2 weeks, no unexpected noises. Whirling sound from 12 bolt rear area. Pulled cover, found a crack in it and very little gear oil inside. No metal fragments or shavings. Cleaned, installed new cover, filled with gear oil. Next time I went on the freway is when I heard the noise from the 203. The 203 is not converted and there is no front drive shaft. Sorry if this is long, but I wanted to give all the details I could think of. Any ideas on where to start is appreciated.