I hear a loud tapping noise in the dash of my 08 Impala when I use the AC then turn it off. I sometimes does it when I start the vehicle and sometimes when i turn it off. It lasts about a minute. The front door locks are going out also. They usually work in the morning but the dont during the rest of the day.

SOURCE: Front differential making a winding noise and ends in a loud thump.

sounds like you need your transfer case looked at and repaired.....that the only thing i can think of...hope it helps :)

SOURCE: IN THE MORNING FIRST START SOUNDS LIKE TAPPING AND KNOCKING NOISE

ck intake gaskets make sure there is no antfreze mixing in oil and breaking oil down if gakets are ok run a factory oil filter and go up agrade in oil such as 10w40

SOURCE: 2002 chevorelet impala wipers not working right

crank arm assy may need to be replaced.

SOURCE: 2000 blazer 4.3 hard starting

You might find this Interesting. go to Edmonds.com and check out the Recalls and Complaints + Service Bulletins for that Blazer. I got lucky and was able to get rid of mine before a lot of it started But, I went through a LOT with it before that!

SOURCE: 2001 Chevy Impala won't start

If the battery has gone dead, it could cause the security system to go into full lock-up mode. If it has locked up the accessories will work after the battery is charged back up , but the power to the starter will be cut off and it wont turn over. You need to do the 30 minute learn procedure. A GM dealer will print it out for you and you can do it yourself. It's very simple if you follow the procedure exact. hope this helps and good luck.