According to the S”A (56:1), one should answer Kaddish in a “loud voice”, but the M”B (56:5) cautions against saying it so loudly as to cause others to ridicule him.
At the minyan I daven, one fellow just belts out YS”R the way some folks holler when at sporting events (lehavdil). It’s quite uncomfortable to hear – for me, at least – and makes it difficult to concentrate.
Since I’m fairly certain this person feels he’s following the S”A, I say nothing. Perhaps he’s right, since the people at that minyan are mentshlach and don’t ridicule those who behave differently. Given that, technically, I guess he’s in compliance with the M.B., too!