Speed Control Issues.

If you are talking about driving along on the highway, mashing the accelerator, and having the transmission drop down to the next lower gear, then that's correct. It's supposed to do that. All vehicles with a standard multi-gear transmission will do that. It allows the engine to operate at a higher speed. The faster the engine speed, generally the more horsepower it produces. It also changes the ratio of engine speed to wheel speed. The faster the engine is going in relationship to the tires, the more torque, or pulling power is applied at the wheel. This combination allows the vehicle to increase speed faster. If you're on the highway and need to pass someone, you don't want to take forever doing it. That becomes a safety issue. You will also see the same thing happen when pulling heavy loads or going up hills too. It gets the engine into an optimal operating zone to maximize horsepower and torque.

cv joints are what normally make that clicking m noise at low speeds ,more prevalent when turning a corner
as to the other faults , run the fault codes as it sounds like a random misfire and may be fuel pressure or injector related

It sounds like you may have a bad bearing on the inside CV half what that is on the drivers side. It is basically a carrier bearing that bolts to the back of the motor that the cv able slides into. Jack up the car and grab the cv axle beside that ceasing and try to move it up and down. Very much play at all and you wind up with that thumping especially under acceleration.

put rear tire in front front on rear see if anything changes

I HAD A SIMILIAR SOUND ON MY 2000 GRAND PRIX, WHEN IT WAS NEW. THEY REPLACED THE EXHAUST SYSTEM, & SAID IT HAD BROKEN BAFFLES IN THE CATALYTIC CONVERTER. FIND OUT WHERE YOUR CONVERTER IS LOCATED. MINE WAS UNDER THE PASSENGER SEAT.

could be your front lower control arm bushings. they are notorious for failing.

Have tires checked, but sounds like possible wheel bearing going.

Hi i have 02 audi a4 slimline. on motorway going 70 mph and to past a car dropped it down to fourth gear probly hit 90mph. Drove for a further hour and a half, and hear a bang under car. car started to loose power. was drivable but hissing noise come out of it when accelerating. Feels like turbo not kicking in. Would this be that the turbo has blown of maybe burst pipe or something. If anyone can share a light on this i would be very greatful. Cheers

Sounds like a wheel bearing that is going bad? Does it change with engine speed or vehicle speed?

Is this noise coming from the rear of the vehicle, if so it "sounds" like a rear pinion bearing concern, this would make a winding noise when under a load(accelerating) and disappear at other times. Keep me posted