Note: It seems that almost any combination of g- or c-, -r-, and -l- with a back vowel can mean 'to growl or rumble', and it is almost arbitrary how many entries one regards this combination as representing. MED has chosen to lump the forms beginning with g- under the single verb grollen, and the corresponding verbal nouns under a single gerund entry, grollinge ger. For the forms beginning with c-, see (for the verb), croulen v. and (for the verbal noun) crolling ger., -- cp. OED's slightly different choices, sv. growl v. and grolling; crowl crowling; gurl and curl. The influence of both L gurgul- (cp. ME gurguling ger.) and F grul-, grol- (gruler) appears relevant thoughout the group.