Dean won’t deny that the sex is great - more than great, actually, because Castiel is flexible in ways that Dean is fairly certain are illegal - but Castiel’s vocal acrobatics are getting to be an issue. Dean’s tried everything, from ordering Castiel to be quieter to gagging him to covering his face with a pillow, but Castiel’s true voice is not a human one and it can’t be silenced by human methods.
For a while Dean wonders if it’s some sort of challenge, or a bizarre mating ritual he’s witnessing; he’s had girlfriends who screamed loud enough to make his ears ring, but for Castiel, sex with him literally makes the Earth move.
The noise is involuntary, as it turns out; Castiel’s true form translates orgasm as religious ecstasy, and all the unearthly wailing and rattling of windows just happens to be the angelic method of expressing it.
Dean wishes it didn’t turn him on, he really does, but as things stand he’ll just have to adapt to getting a hard-on any time he hears a window frame vibrating.