im the mate with which he swapped BOVs with and the mounting locations are similar, in the stock BOV location
when we removed the BOV from the R34, we had to twist it so the spring became more loose in order to reach one of the bolts and i left it as such on my car, the 33.
i would say the difference in sound is now caused by the BOV not being as tight as before and it opens easier and louder than how it was on the 34..
sorry for not explaining this to you earlier Erik