We had our bath tub & shower valve replaced about 18 months ago. The valve had corroded and was leaking behind the shower wall. It also lessened the water pressure when turning on the shower lever. Just recently I've noticed a very slow drip coming from the bathtub knob, a drip every 5 minutes or so. Is this a recurrence of the valve issue we had or an entirely different problem?

You are not stating brand and type of your faucet. From our question, I assume you have 3 handle faucet (hot, cold and diverter handle )...

Drip from the handle would indicate loose packing nut. It is relatively easy fix: remove handle and tighten up the packing nut. You cannot miss it: it is the only nut under the handle. Give it 1/4 turn. Leak should stop. Handle will be little stiffer to turn for few days now but it will work itself in...

More information on my initial question. The brand is Temptrol Pressure-Balancing mixing valve. Its one of those single shower/tub, triangular knobs with the bath to shower valve beneather the water knob. I don't know if this changes the initial diagnosis, but wanted to provide more information.

Use Milo's original advice. A drip at the handle is packing, a drip at the shower head would be a washer or cartridge.