The word "feeler" typically refers to someone who is testing or gauging a situation or someone's reaction before taking further action. It can also describe a part of an insect's body that helps it sense its surroundings. Full definition
You can put feelers out there, gauge a reaction and if you come under suspicion for your hinting ways, there's no damning evidence because you didn't actually say anything. (theeverygirl.com)
If you're interested in dating with a couple or having a threesome, you simply have to put feelers out there to see who is up for such a relationship. (bisexualdatingsites.biz)
as for the «gran of salt» the Toyota tech manual (by Toyota, not Haynes) says to measure the clearance with feeler gauges and replace any lifters that are out of spec (mechanics.stackexchange.com)