GM-GMC: GMC Horn and startup, fuel pressure gauge, fuel issues

Question
2000 GMC p/u Sierra 350.  
1.  Horn abruptly stopped working.  All interior/exterior lights, radio, door locks, windows still intact and working.

2.  Trouble starting - seems to not get enough fuel.  Happens intermittently.  For example - will start normally in morning (first start), but subsequent starts it will sometimes will die unless I feather the gas pedal.  Fuel filter replaced and added MDX? gas treatment.  Some hesitation when accelerating - again not regularly.  Used p/u that I purchased 12 mos ago.  Hear whine that is not associated with rpm on driver side rear (fuel pump?) - don't know if that could be related.  Thanks.

Answer
1. I would first check your horn fuse.  If that is ok, locate one and remove one of the connectors on your horn, there is one on each side near the radiator area.  Have someone press the horn pad and check for power and ground.  If you have power and ground you need a new horn if no power and ground, locate the horn relay and swap it with another with the same part number and see if that works.

2. Most of the 2000 p/u trucks had either a 4.3/4.8/5.3 engine not a 5.7 however, you sound like you are on the right track.  Install a fuel pressure gauge and check your pressure and see if it holds pressure overnight.  If the fuel is draining back to the tank it would cause a hard start or if the fuel pressure was insufficient it would also cause the symptoms you are describing.  As for the noise, it could be your fuel pump which would make sense if you are having fuel issues.
Hope this helps.