Crackling Sound When Shifting to D and R

Hello, good day. I have a 2002 Peugeot 206 automatic transmission. I just bought it. I hear a hissing noise coming from the engine when I shift into D or R. This noise wasn't there when I bought it, but I cleaned the engine with engine cleaner. I don't know if it's because of that, but the noise has started. Is this normal? The noise stops when I shift into neutral. How can I fix this problem?

It's normal, sir. It's an indication that the solenoid valves are working. It makes a buzzing sound like a bee. If you don't hear the actual sound, then don't worry.

After I replaced my solenoid valves, I started hearing the same sound. If there was a problem, my mechanic would definitely tell me.

I'm also getting that sound. Sometimes when I shift to N position, a line (-) appears on the screen and then the engine fault light comes on. The fault code is u1209, meaning there is no communication with the gearbox computer. There's no problem with gear shifting. What's the problem?