Cold starts cause engine to make clicking sounds.

I have a 94 pickup with a freshly rebuilt 350 motor. On cold mornings (below 30 degrees) the engine makes a clicking sound when first started and for the next minuit or so when I start driving. When it's really cold it sounds more like a diesel than a gas engine, but just for a little while and then it runs like new. Throughout the winter it seemed to consistently get worse. I had it back in the shop once and I'm not sure that the mechanic did anything other than change the oil. I called him again and I'm going to bring it in this monday. He said he thinks it's a bad check valve. He thinks the oil is draining out of the system when the engine is not running. Does this sound right? Would temperature have so much effect on this problem that it seems to dissappear when it's warm outside? Wouldn't the oil pressure come up slowly like when you change the oil? I have oil pressure right away. My warranty is about to run out and I have no way of knowing if it's fixed until it gets cold outside. Any advice would be greatly apreciated!