Noise when starting truck

Hi.
When I start my car it makes this weird jerking noise
and sometimes doesn't start give it 5 mins and start
it and makes the same noise. I took it to a friend
that's a mechanic and charged the started and
water pump and still have the same problem. So
took it to the Cadillac dealer they did a computer
check and said it was my battery so got it replace
and it was good for a month and the same problem
started again and now it's at the dealer and they
don't know what the problem is.  But they called a
day later and said they got a code from the chip in
my key so are they correct this time or is it
something else cause the problem? Please help?