I think it was in Wodehouse that I came across a jocular use of gruntled and I always thought it was simply a gag on PGs part. Which of course it was, but now I find that there is indeed a gruntle, behind disgruntled, although sadly it does not have the meaning that Wodehouse playfully ascribed to it.
Etymology: < dis- prefix 1e + gruntle v. frequentative of grunt n.
trans. To put into sulky dissatisfaction or ill-humour; to chagrin, disgust. Chiefly in pa. pple.
1682 H. Care Hist. Popery IV. 79 Hodge was a little disgruntled at that Inscription.
Etymology: < grunt v. with diminutive or frequentative ending -le suffix.
1. intr. To utter a little or low grunt. Said of swine, occas. of other animals; rarely of persons. Const. against, at. Obs. exc. dial.
c1400 Mandeville’s Trav. (Roxb.) xxx. 135 Þai..spekez noȝt, bot gruntils as swyne duse.
2. To grumble, murmur, complain.
1591 R. Bruce Serm. Edinb. i. sig. B4, It becommeth vs not to haue our hearts heir gruntling vpon this earth.
Grunt BTW is, unsurprisingly, echoic in origin and was originally descriptive of the sound a hog makes.