Starter "Pops" but no crank
 

Ok so my wife had a 2001 Civic EX with the D17 and yesterday it suddenly wouldn't start, I tried jumping it with my p71 (220 alternator) and nothing, pulled the starter out brought it to parts store to be tested before buying a new one and it worked for them, put it back in this morning ND everything worked fine for almost 18 starts throughout the day, again tonight she called me saying it wouldn't start again. And all it does is one pop when I go to crank it over but won't spin, anyone have any ideas before I go buy a brand new starter And to what it may be?
2001 Honda Civic EX
Automatic Transmission
Battery is good
Do not have mutli meter to check voltages but buying one in two days when I get paid

Re: Starter "Pops" but no crank
 

Smack the starter with a small beatandpound, see if that gets it to crank?

Twist the key 50 times to see if it occasionally cranks, like bad contacts in the solenoid? (starter makes a clunk, but no crank)

Loose fit (and thus poor connection) on the starters small wire spade terminal, due to age and vibration? (wiggle the wire while someone holds the key in the crank position)