I have a few questions about noise gates. I'm currently using the noise gate in the line 6 m5 with the decay at zero and the threshold at 30%. My signal goes through a Maxon od(boost only) to the m5 to an AMT preamp to the sound card.

Whenever I turn the boost on and finish a lick(and roll the volume down) there is woshh sound which lasts for about 2 seconds and fades. Increasing the decay makes longer. What causes it?Is it because the m5 doesn't react fast enough?

I was also thinking of getting an ISP decimator (not the G string) but I noticed it has no decay knob. How does the decay work in it and will it cure my noise issues?

Thank you
There is tc delay after them but it doesn't produce any noise. Each pedal is being powered by an isolated output. I get no noise at all when using the amt by itself, but engaging the boost makes the noise apperent (which is expected) . My issue is the wosh sound the m5 does after turing the volume down. The guitar is a jeff loomis signature and the electronics compartment is shielded so are the cables.
Often the back panel is shielded, but not the cavity. Are you sure?

Cables are always shielded, it's the nature of coaxial cable - but that doesn't prevent them from being noisy. Try different cables.

What power supply? Not all bricks are isolated.

Here's some helpful info:
Skip down to the part about Line 6 stuff.

Try inserting this between your Maxon and PSU, see if it improves:
A 100nF instead of the 47nF would work well too.