The service engen soon light is on and the truck shutters between 30 and 40 mph what could that be. It also seems to be loosing h.p

Take to an Auto Zone store or other store that gives free code scan check. Some auto makers put built in alarms into the ECM to be reset by manufacture, now outlawed practice.

Have the crank position sensor checked. This sounds very familiar.

Dave, the knocking noise may end up being unrelated. As for the shutter/hesitation, depending on the vehicle, it may be a filter/in-line valve. Check the pressure hoses and the outlet of the pump. Certain pump designs will have, what seems to be an extension coming off of them, a pressure valve/limiter..

is your check engine light on it could be anything from fuel to emissions

Why is the check light on for months?
As soon as you get a light on, you have the possibility of a stored code, and a place to start your diagnostic testing, immediately.
After 30 years of electronic systems, installed on vehicles, I don't see why people don't understand how their vehicles function.
Take it to a repair shop, as soon as the light comes on. Then you find out what may be the issues, where as waiting, could possibly cause further problems or damage.

If you get ck engine light on , you have a cyl. number
that misfires , ck vacuum leaks on intake manifold around that area aspecially intake manifold gasket.

Hi:
You need to have your code scanned to see what tripped the light. It could be as simple as a loose gas cap, or hundreds of other reasons. Most local auto parts stores will scan your code for free, so you will know what is going on...

run scanner for trouble codes, some auto parts store will do this for free , check for loose, broken or disconnected vacuum hose

I too have that problem. I own a 2005 GMC Yukon. Shutters between 40 + 55 mph. I thought for sure the stumble I felt was a plug or coil pack because it only does it under a load. With A/C on the whole truck shakes! But it wasnt the plugs or coil packs. Answers anyone?

disconnect the battery for 5 minutes, that will reset the check engine light and clear the code you set..