Volvo: 1999 volvo S 70 91000 miles, volvo s 70, air bubbles

Question
Tapping sound coming from engine that Comes and goes.

Valves? thicker oil? better gas?  

Answer
Jeff,
Lifters are possibly not getting enough oil and ticking. The oil pick up seals on the oil pump get dried out and shrink causing the pump to suck air and not pressurizing and delivering the oil. Run the car at idle after warm remove the oil cap carefully and see if you see air bubbles. If you do seals are bad. If not lifters might be going out.