Car - 940GL (non turbo), 104K mileage

Backfiring or cracking sound without car jerking when on the move. The backfiring sound only comes when I let go the acceleration at mid - high rpm. It sounded like cannon usually four or five times being shot. Fuel consumption noticeably higher.

My mechanic has done the followings:

1.Changed the spark plugs

2.Cleaned the throttle head and did some adjustments

3.Changed the air filter

4.Changed the mid and rear end of the exhaust pipe

5.Performed a top overhaul of the engine to clean the carbon off the pistons and changed some valves inside the engine

6.Checked the diagnostic read out but no error codes from the ECU

7.Tested the engine knock sensor and found it okay.(he used another 940 knock sensor on my car for the test)

8.Dare not clean the injectors because these are expensive items. (I told him not to change them if he is unsure. Anyway the diagnostic tested okay and there was really not jerk to motion.)

What else is wrong?

Please help!!!

Another problem. Loud humming sound coming from gearbox (my assumption) when I bring the engine speed up to 1500-2000 rpm. The sound is usually lounder when climbing a slope although the sound can also be heard when vehicle is stationery. I was told it is the gearbox, possible some bearings near the fuel pump. I need to confirm before changing anymore parts. My transmission fluid is replaced with oil treatment but the sound still persists. What else to try? Please help!!!