1998 auto transmission problem

I have a 1998 Explorer Sport, 2 wheel drive with auto transmission. The problem is a clunking noise that is heard after the trans shifts
to second gear. Seems that the transmission shifts smooth into 2nd but a couple of seconds or so after it shifts it makes a klunking noise. Looking at the tach the engine s rpm stays the same. The engine is a 4.0 SOHV. The transmission shifts the other gears ok. Any help would be appreciated.
Thanks in advance
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