Pinging sound only when acclerating. Doesn't do it when at idle or coasting.

You did not mention when this pinging occurs (all the time? idle? lugging under load?). If it is in fact in the engine, it is most likely a timing issue which can get complicated because it could be caused by a problem with the distributor, timing chain or chain tensioner, one or more faulty engine sensors, etc.

However, there are other causes of this noise. Most pinging noises at idle or low RPMs are not usually in the motor itself but rather from rotating parts that periodically touch something, and often happen when something comes loose or is bent, bearings get worn, or something made it into the engine compartment that shouldn't be there (a tool, a rock, etc.). Typical examples are worn out HVAC fan, cooling fan or AC compressor clutch bearings; a cooling fan blade touching metal or a loose shield; and so on. If this is the case, you should try to locate exactly where this noise is coming from - a stethoscope from an auto parts store may help.

You provided no info on the vehicle motor,so
I would assume you may have an Air Idle Solenoid
next to the throttle or on the intake manifold,that you
can remove & try cleaning or replace.

When was the last time you had a tune-up?

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

whats the make and year of your car? Mine does the same and its a 2000 model toyota harrier 3000cc. I have changed the idling sensor, airflow sensor and had the throttle sensor checked and the fuel but nothing seems to work.

had the same problem.
got water on the computer located under the drivers seat.
had to replace computer at a cost of zar6500.after fighting with toyota they replaced free of charge. only cost me zar850 for labour.

If it runs fine and the idle is the only problem you could try idling it up some with the idle set screw on the throttle body it sounds like it may be a little bit out of time those toyotas have a fine lins on the timing.

For questions 1 & 2 it sounds like either a shift solinoid bad or a gear itself i would have that looked at by a transsmission shop soon and for 3 usually when the tach jumps up and down when idling you have a skip like a plug or wire i would do a tuneup on it and that should take care of 3.

could by worn piston rings or timimg

sounds like your lifters or valves are rattling!

transmission is bad