Plastic crackling sound on + and - buttons on Key 37 normal?

Hello, i just bought a (secondhand) Key 37, pristine condition, but i'm noticing a "plastic foil crackling" sound when i press the + and - buttons underneath the data-wheel. The other buttons sound normal, with a plastic clicky sound and feel. But these 2 buttons sound different, more "fragile". It's especially noticeable when not pressing these buttons, but moving them around without despressing them, i hear some kind of crackling foil/plastic sound underneath them. They work normally.

Is this normal for a Key 37?

Tnx! Olaf