噜 is mostly heard inside compounds: 呼噜 (snore), 咕噜 (mutter, gurgle), 噜苏 (long-winded). Online and in cute speech, it sometimes replaces 啦/咯 as a playful sentence-final particle, e.g. 走噜! ('let's go~'). Standalone, it's almost always onomatopoeia.
Left mouth radical in its grammatical-particle role. When 口 sits beside a phonetic with no concrete meaning underneath, it usually marks a sentence-final particle or a soft murmured sound, just as in 啦, 哩, 喽. 噜 is the slack low murmur for casual speech.
Right 鲁 supplies the sound lǔ → lū with a tone shift to neutral / first tone. Pure phonetic; the 'blunt, dull' meaning of 鲁 and its association with the ancient state Lu have no bearing here. Picked solely because the l- onset matched the murmured particle being written.
