Noise Under Dash

[mwmst]

The is an intermittent noise coming from under the drivers dash. The best way to explain it is it sound like someone thumping the spring on the top of a 6V (deer feeder) battery. It does not matter if the truck is moving or parked. It will go away for a long while and just magically return. Any ideas on what it could be?

 

84K miles, so warranty not an option.

 

Thanks in advance,

 

mwmst