invest in some good vibra-stop pads that go between the engine and hull.

Mattoo - 4/strokes are so differnt in noise characteristics than the 2/s that they can still seem noisy.  Need to jump back on a 2/s for benchmarking.

In saying that I have found the Suzukies I have been on to be a bit more noisy (gutteral?)  than the Yammys when under the hammer.

There are variances even with same engine  -  last November I swapped out a 700hr 2006 F150 Yammy for an new 2012 F150 Yammy on same boat Surt 6.1HT with same mounting pads and  the new engine is definitely noiser than the old unit.

Also the Surt Hard Tops can be a resonance box.... the local dealer was steering folk away from the DF140 on the 6.1 HT's in favour of the DF150 due to resonance .  And the DF140/115 used to be twins - do not think that is the case now though.

Why don't you PM Davo and discuss - he has run his DF115 in on the 5.5HT .

my 2 stroke use to hum like hell, found out the virbra stop did work but not as well, so took it off, and found out the bolts did not have washers as ya call em on them under the main metal washers, so was metal on metal. but changed them and things was sweet as. so was sweet as when metal on metal was eliminated.. 

if you have fiddled with the plug wires & perhaps put one cap on the wrong plug it will through the timing out too & the motor will vibrate

with regard to Durbans post:

Is the performance ok? If it isn't performing well or is thirsty it could be a misfire/tuning issue. If a plug cap is on wrong it will run terribly!

You mentioned you have a 25" extra long shaft Suzuki on your 5.5 Surtees. Curious is that what that transom is design for?

I had an anoying vibration type drumming at the cruizing revs on my 5.5 workmate, folding top with a honda 90. If you held on to the steering wheel and moved your head back as far as you can it dissappeared so it must have been resonance in the cabin, considerred lining it but bought a 6.1 with a F150 instead. Nice

What about trying different propellor hub brands?

Sorry for being slow on the feedback. Ive gone from a 90hp to a 115hp (both suzuki 4 strokes) on my 5.5 hardtop. The difference in handling and performance is awesome. I had vibration issues to start. I have a vibra stop pad (always had one). It seems to help. I wasnt happy with the vibration noises so got my dealer to drop the engine one hole, tweak the bolts up a bit more and i put in some of the insul tape for window draughts in on the rear battery compartment and that pretty much killed the noises. I still get a low drumming through the hull at very low revs when the engine is in gear but im used to it now and figure itll help raise big pelagics...hope thats useful.

try giving john butler a ring at butler marine in CHC, i remember he said to me that they market anti-vibration products. worth a call.