GMC Denali 2003 Problem

I have a GMC Yukon Denali with the following VIN : 1GKFK66U13J_ _ _ _ _ _

The problem i am facing is when the car speed exceeds 60, it starts to cause sound of firing from the silencer continuously. I took it to repair shops for diagnostics but there seems to be no DTCs present.  

The repair shops have already tried the following:
1. Replaced the crank sensor
2. Replaced the fuel return valve
3. Replaced the spark plugs
4. Cleaned the fuel injectors

All this went in vain. If anyone on forum help me figure out the actual problem it would be appreciated.

Thanks in advance.  

i took a look over at alldata for TSBs on this...  only one for hot clicking noises from the exhaust.. while idling..

http://alldatadiy.com/TSB/23/0323bLj1.html

first  i would scan for trouble codes...  something in the PO300 to P0308 code would indicate a misfire and point you toward a cylinder that is going bad..  post any codes you get...

at this point.. i would probably mark the coils for position...   pull them and the spark plug wires...

examine the coils carefully for signs of external high voltage leaks... white dots/marks in the black plastic .. dust circles with a discolored area in the middle..

i would also look carefully into the ends of the spark plug wires...  make sure that the metal terminals inside were not pulled off the wire.. this happens a LOT on other wires...  OHM them.. set the ohm meter to 200K ohm scale..    1,000 ohms per inch MAX....   since these are short wires..  you are probably going to get readings on the meter display of 3 or 4...  (3,000 to 4,000 ohms)

you might after inspection.. of the coils.. just install a fresh set of spark plug wires... please use a bit of dielectric tune up grease around the inside of both boots... but only on the inside of the boot.. not where the metal contacts make contact..  this will seal any high voltage leaks that might be causing you problems..  

I changed the injection coils set and my truck is all good now. Thanks a bunch for the help