What I'm looking for is the Japanese equivalent of the sound of taking off a helmet, particularly a full-head helmet, like a racing helmet or a motorcycle helmet. For the life of me, I can't find any manga panels to reference. The closest I've found is スッ but I think that only applies to fabric, like shirts or jackets.
スッ can be used if the helmet is taken off smoothly, without friction. サッ may be ok too.
スポッ is better if there is some momentum when taking off the helmet. The word describes something fits in tightly, and for the reverse direction (putting out).
- 帽子をすぽっとかぶる (source)