Hello,
2007 Expedition: 3 days ago when I started the Expedition in the morning I noticed a new and distinct hum or whine from the engine, only at idle and very low RPM. After driving a short distance when the engine came to operating temp there was a burning smell coming from the engine compartment. Hard to describe but maybe rubber or plastic burning smell.

Don't know if related but the last 2 mornings the battery has been a bit weak in turning over the engine as well - only when cold. Alternator gauge shows charging and serpentine belt appears okay from a distance.
Any suggestions?
Thank you.

 

It's good that you are paying attention to the sounds and smells coming from your vehicle.
Most of the time anything that rotates like alternator,a/c compressor, water pump, idler pulleys timing chains to name a few, could be telling you early, that something is not right.
To top it off the additional offering of the unusual fragrance coming from the engine compartment is another early notice something is not right under there.
If upon your own inspection you cannot see any reason for the notices , it is time to have an experienced shop check it out before it get expensive to correct.
As you can see there is only one way to diagnose the problems and that is by actually being there to see, hear and ,yes, smell check for the source. It could be something simple and possibly if ignored become something other than simple.

 

Well, looks like you are getting some early sign of engine problems, get it checked to some experienced professionals.