2001 ford explorer sport clicking sound

My dad passed away recently and I inherited the
2001 explorer sport (automatic) that he had
purchased a few weeks prior to his passing. I
drove it 240 miles back to where I live and it ran
fine except I noticed a clicking sound (Like a
playing card in a bike tire) whenever I pressed the
gas or was idling, and it struggled going up an
incline especially going over 45mph. The temp
gauge always sits about in the middle but as soon
as the road inclines and you're at a high rate of
speed the temperature needle goes all the way to
the top and let's up once you let off the gas and
start down hill (Don t know how accurate the temp
is, my uncle's said the needle was wrong). Oh, and
the check engine light just came on today and it's
running really rough.