Genuinely fascinating area for this kind of report. The Olympic Peninsula has a long history of large unidentified animal sightings going back well before the whole Bigfoot thing got popular, and the rainforest around Quinault is dense enough that something sizeable could theoretically move through it without being documented properly.
What's catching my attention is the "moving through" description. A lot of misidentification cases involve stationary sounds - branches cracking near a camp, that sort of thing. But something actively travelling through the forest at night with enough weight to be audible over distance, thats a different category of report entirely and worth taking seriously.
I've spent time at some of the older haunted locations around Wales where the grounds border heavy woodland and the acoustic properties of dense tree cover genuinely do strange things to sound, so I wouldn't dismiss environmental factors either. Sound channelling along watercourses near the lake could be amplifying something much smaller and making it seem enormous.
That said I don't want to just explain it away. Has anyone managed to get any audio recordings? Even rough phone footage of the sound pattern would help establish whether this is rhythmic movement or something more irregular.