I have a 2002 F-150 and started making terrible noise when turned on heat/ac. The fan cage was full of acorns and pieces of insulation in it.
How can the squirrels get in there? I have looked and see no entrance,maybe just don't know where to look.
Any suggestion????

 

Probably deer mice or some type of mouse. They filled the hood insulation right above the battery with acorns in just a couple of days on mine. Flip traps put an end to it. They can get into anywhere.

 

I used to work in an A/C shop. I can't count how many heaters we pulled apart like that. Occasionally, the heater would be turned on while the rodent was still in there.

Trust me, you are lucky you didn't.