I've had quite a bit of success but its relative as we have a Perkins 4203 so there's a lot of noise to start with. Its an on-going project. What has worked so far:
Roof flashband on doors - huge diference and its still on after 4 years. Do it properly, clean and prime the press on with roller.
Roof flashband on bulkhead and footwells - still on tight after 4 years. Made a a big difference but not as much as on the doors
Finding and sealing every (and I mean every) hole or pentration in the bulhead. Used gutter mastic as it stays flexible. Made a worthwhile difference.
Sorting out the bonnet rubbers, rad strip and hinges, suprsingly effective, a lot of noise must have been the bonnet rattling.
Putting carpet on the doors and rubber mats on the floor (the cheap fitted shaped ones). Helped, doors now absorb sound.
Headlining, made from 3mm correx glued and stapled to sliver bubble insulation and carpeted. fixed on with ally stips screwed into fixings under the roof strips. All removable Killed a lot of reflected noise.
Rebulding the engine - helped a lot, should have done it sooner!
Connecting the radio to a pair of Army suplus tank headsets - fun and works to some extent, need better speaker units. Still not linked it to a Karoke unit so we can use the mikes to speak, at the moment you have to yank the cable to get attention.
Keeping the vents shut, pity in this weather. suspect lining the bonnet would help with this.
Cost? Not much, carpet for the whole inside about £50, flashband £10, mastic £5 Correx £12, mats £25, ally stips £6 odds and ends £20. Headsest were about £12 each
We nearly always drive with some form of headset or ear-defenders but we are doing 55-60 for up to 100 miles at a go when we go to an event.
Backwards step - roofrack, adds to the noise.
I think we are still gettng noise up through the seat box and the covers over the underseat tanks. Centre toolbox is lined.