1996 Chevy Blazer Engine tapping noise

I recently had a full tune-up and oil change with a engine flush. Well 3 days after i got it back from the shop my truck starting making a tapping noise and the oil pressure gauge started acting abnormal. I would go start my truck in the morning and it would start up and run great, and the oil pressure would be right on. This problem only happens after it runs through the warm up cycle and is at normal operating tempture. The tapping is coming from the bottom of the block. I have been told things like the timing chain is expanding and hitting the cover after engine heats up. That the oil pump is clogged after the engine flush, and main bearings. I little info in the past 3 years i have owned this truck i have been a fanatic about oil change and tune-ups or anything that came up. Because of this my truck has always run outstanding. Any real professional advice would be helpful, thankyou