It is probably due to the musicians' bowing on their instruments, if it's what I think you're talking about. Surprising that it comes through so well on a recording, as you rarely can get that effect on record, but live it is very noticeable and you're right, this piece seems to show it up in virtually every version I have as well. Almost like a grunt from the orchestra. The musicians have to dig in for the climax, and its physical effect is to cause an audible noise apart from the music.