Tranny Slipping

[1991Explorer]

I am not exactly sure what this issue is, but it hasn't prevented the truck from driving. When I put the truck in drive it take a while for the transmission to engage. The motor revs, then you hear a clunk (not too loud) and the truck is in gear. This is happening more frequently now than before. I was told by someone they think it's the cluth, but I don't notice it slipping when I floor it, nor is it having issues shifting. Any suggestions?

 

[MrShorty]

As with most auto transmission issues, the first thing to check is fluid level.