I bought this house with an in-ground swimming pool 2 years ago.

Never had any problems until now.

It's a pool spa combination. About 10,500 gallons. Two skimmers. Two drains
(pool and spa) heater, floating chlorine dispenser, sand-filter and Polaris.
Pool is about 35 years old.

Just recently it started getting air bubbles coming out of one of the spa
input jets. I can hear some air going through the plumbing near the
filter/multi valve. I don't see any bubbles going into the main pump filter
basket.

The sand filter's pressure gauge indicates a small decrease in pressure
(approx drop from 17lbs to 16lbs) but when any leaves get in the pump filter
basket the pressure drops dramatically. (past 10 lbs)

I've also noticed more sand in the pool from the filter

Questions:

How can I track down the leak?

Could it be the valves gaskets?

Does a worn spider gasket on the multi valve cause sand in the pool and
could this be where I'm getting the air bubbles from?

Could the air be coming from a pump gasket?

TIA