Hot water Circuit Noisy

I have a Y plan heating system and when the hot water is heating up on its own the water is backing up via the vent pipe and depositing water into the FE tank. The speed is set a 1 on the grundfos pump. When the Rads are on their is no water being deposited in the FE tank via the vent even if I put the speed upto 3 on the pump. Although one of the rads does bubble quite badly.

To try and get rid of the noise and problems Ive so far had three engineers who are puzzled by it and as a last resort I go the system power flushed and the problem still exists.

Does anyone know what elese I should look at without spending more money on a heating engineer.

If you have had engineers out then I guess what I'm about to say will already have been covered.
Has this just started happening ? If so, had anything changed when it did ?
What is the rate of overpumping (water out of expansion pipe), dripping, dribbling, running ?
What level of water have you got in the F&E tank ? as set by the ball cock.
How high above the F&E tank does the expansion pipe go ?

Reason for asking the latter questions is:
The static level of water in the F&E tank should be just enough to cover the outlet pipe at the bottom of the tank, adjust with ball cock to achieve this.
As this is a gravity situation, the static level of water in the expansion pipe is the same as that of the tank, therefore, if you have a high level of water in the tank (and pipe), it will take very little expansion for it to come out of expansion pipe, particularly if the pipe doesn't go very high over the tank.

I had this situation with too high a level of water in the tank and a low (only about 30cm high) expansion pipe.
I set the tank level as described above, increased the highest point of the expansion pipe to almost 1 mtr above the top of the tank, bringing it back down to the tank again in an upside down U shape and no more over pumping, including higher pump speeds.

As I started, I guess these obvious things have been checked if you have had 3 engineers out, but just in case..............