Even though I've never been to LM I've experienced a lot of endurance races.
Spa is pretty good also. Like the exit of Rivage on the outside, or opposite to the pit exit.
No need to talk about the exit of La Source (I can't tell you how I got there and an amateur photographer
)
The 333SP's weren't that loud. But the sound was superb. I also noticed huge differences in the sound between those lovely cars. In ISRS there ran half a dozen of it.
The Panoz LMP-01 is probably on of the loudest I've seen.
Also a good call is the Jim Gainer Mugen.
The GT3 RSR's are may be not loud but they have a strange frequency that fight their way into the head
(if you're 5m away from them)
But now being in my 14th year of watching races there's not much that impresses me.
The only thing were I covered my ears were the blowers during the German500.
Those pick-ups with a turbine on their back for drying/cleaning the track.
The Champcars themselves were pretty loud too, but it was rather hearing them all the time than their loudness. The turbocharger reduces the sound pretty much,