knocking/ticking noise from hot water pump

I recently replaced some radiators in my house. Since then the circulation pump makes a constant knocking or rapid ticking noise, and the boiler sounds like its got air in it.

the pump is a Myson U3C252 unit 3 dual speed. I've tried loosening the black nut in the middle of the pump, no air comes out just dirty water. i did this when the pump was off and didn't completely remove the nut. This made no difference.

I've noticed at the back of the pump a blue lever that looks like it could move clockwise or anti clockwise but don't know what its function is so haven't tried.

The boiler is a potterton Suprima 60

i've got 8 radiators in the house, 2 of which are bar rads in the shower and bathroom, the rest are singles all average size, except the one in the sitting room which is a double. i've taken one rad out of the system and put this double in to complement - the sitting/drawing rooms used to have one each but now there's just the one double.

I've recently bled all the rads.

can anyone suggest solutions re these noises?

Many thanks

The lever on the back of the pump opens & close's a by pass in the pump housing. (bad idea) if this is open it will reduce how effective the pump is. if you can see behind the pump u will see at either side of that lever is a faint + & - set it to the +. Did u add inhibiter to the system when u filled it up . If no. then u must. How long is it since u filled it. they can be noisy 4 a couple of days. run heating ony 4 a couple of hours on max pump speed.

Don't move the lever - yon pump's probably 20 years old now and it could leak.
Air bubbles make the noise, almost like fine gravel going through the pump

I haven't touched the lever, no didn't put any inhibitor in, what does this do and where should i add it (I am very ignorant about all this I'm afraid). will it get rid of the air?

thanks for your input