Leaking/cracked exhaust manifolds. Never heard a v6 knock sensor gone bad (the sensor itself, wires can be broken due to careless placement). Does the sound appear only when running cold/hot?
Engine temp doesn't appear to make a difference, but air temp does. Nice hot humid day and it purrs. Some days it runs quiet,
but then you pull up at a junction and when you pull away the rumble is back, but only when accelerating. Cruising is always quiet.
Pretty sure it's nothing cracked or leaking, not that sort of sound, and 99.9% sure it's nothing mechanical as you can rev it when stationary without any rumbles or rattles. Cables look OK, down as far as I can see them. I,ll have to get underneath and have a look at the bottom end of the cables. I wondered about the MAF as knocking can also be caused by the fuel air mix, but the
sensor should adjust for that.
Forgot to mention it's only done 61,000, so still all nice and clean.
Pretty sure it's northing serious, but it's a noise that I notice and would like to stop.