Suddenly scared during walks

I have an 8 year old jack russell / chihuahua mix. A month ago he suddenly started exhibiting a new behavior during his walks. Normally, his walks are about 20-30 minutes long, and gets two per day. Now during almost every walk, he'll get scared by a loud noise just a couple minutes into the walk and want to run home. Things like distant cars backfiring, a screen door slamming, someone hammering, or a motorcycle driving by etc. Not loud or close, but low pitched. A fire truck siren doesn't bother him. Our neighborhood isn't any noisier than usual, and sometimes I don't even notice whatever scares him. He doesn't tremble (like he does for fireworks) when he gets home, and will go out again later - he's never reluctant to go out.
I can't think of anything that would have caused this behavior change.
What can I do to make him confident on his walks again?
This is also a problem because if he doesn't walk enough he won't use the bathroom, and he won't do it in the yard at home.

Asked by Member 1144925 on Dec 12th 2012 Tagged fear, walking, suddenchange, anxiety in Fears & Phobias