Space Marine Stealth
When power armour is on, Space Marines receive -30 to stealth rolls in an environment where their audio signature would stand out.
Situational modifiers apply as regular. Sneaking in a loud environment might negate the effects of the power armour’s sound, whereas sneaking in pindrop silence would make it more prominent – and possibly impossible to sneak without first turning the power armour off.
Power armour cannot be turned off, but it can be taken off to avoid the penalties associated with it.
Of course, the unpowered variant, scout armour negates these penalties.