Stopping the overflow noise i my toilet (ideal standard type)?

Had a roaring noise upon flushing but have since fixed that but now i have a couple of issues with the toilet, it's an ideal standard with to grey flush buttons...

1. the overflow noise stays for a while and i would like to know how to stop that AND
2. Upon removing the cistern lid, i accidently depressed the plastic centre piece underneath but don't know how to raise it back to the original height as it seems stuck fast by the design of the notches running down the length of it.

Any info/help is appreciated