I used a 0.010" (0.254mm) feeler. I have used a screwdriver as a stethoscope before without hurting myself (yet), but never heard of the hose pipe one, i'll have to give it a whirl this evening. I think it'd be best if I just take off and inspect the rocker assembly, that way I can see what I'm dealing with and check that the whole lot is sound and was torqued down properly.