Yes sir you are right about it.
OK, in that case, lets start here:
Accept that the derailler cage never changes angle. It simply moves parallel to the cassette sprockets. It keeps that perfectly parallel position from gear one to gear whatever (9/10/10/11/12 or however many gears you have). Conform that for yourself if you don't see what I mean.
However, the bottom run of the changes angle significantly. Only in the middle gear (say gear 5 out of 10), will the chain's bottom run and the cassette sprocket be parallel and in line. In all the other gears, the chain line and sprocket plane are out of sync.
In the lowest gear (largest sprocket at the back), that line between pulley and chain is at its worse. The chain wants to pull across the pulley. The pulley points directly forward but the chain pulls to the right. In other words, as the pulley rotates and a tooth falls into the next available space in the chain, it hits the chain's sideplate and then slides down into the hole. That's why a chain in that position is noisier than one in the middle gear. There is mechanical interference.
To minimise this mechanical interference, the derailler manufacturer gives that pulley a special shape. The pulley's centre line is offset to the right. In other words, the chain doesn't run perfectly in the centre between the two end points on the pulley's axis. Now, if that pulley is installed the wrong way around, it accentuates the problem rather than solve it. That's why the pulley has a directional arrow on it. It is an indirect way of saying Left and Right. It has nothing to do with directionality of rotation but with positioning the offset in the right place.
I suspect your pulley is installed the wrong way round. Have a look and let us know.
The reason it works perfectly in the reverse pedal mode is because then the incoming chain is fed directly from the pulley above it, which is perfectly in line. The noisy problem is not at the top instead.
I hope this explains it somewhat.