88 Camry Transmission noise

I've got an 88 Camry 4 cyl with an automatic trany. About 30k miles.
Bought from an old lady who only drove to church etc. 

Was driving today doing about 45 and experienced that noise you get
when you drive over loose gravel and it kicks up under the car. This
continued as I was driving along the road.  When it didn't go away,
even when I pulled up to a stop light, I suspected something
mechanical.  It sounded like it was coming from under my ****fter.  I
moved the ****fter into neutral and then back into drive and the
noise went away and hasn't come back.  I haven't been over about
25mph since then though.  

Anyone experienced this ? 

Should I get an inspection or do more test driving ? 

No leaks I can see, fluid looks clean and red. 

Haven't replaced the fluid, sounds like 30k is time.