I recently noticed a noise somewhat like "tsu..tsu..tsu..." when my pick-up reaches about 30 kph. Para siyang tunog tren na pabilis ng pabilis as I accelerate. The speedometer needle also somewhat vibrates erratically. Above 40 kph speeds ay wala na. The same is below 30 kph. Walang wiggle sa steering wheel. It happens on paved roads. I already had the underchassis/driveshaft parts greased pero nandoon pa rin. Bago na din transmission/differential gear oil at factory specs. Ano kaya ito?

Thanks.