Re: Noise/vibration high load low rpm
THEN GET MODDING! kidding...
Cat's can do a lot of things when they fail. The worst, besides a possible rotten egg smell, is leaving you stranded from it clogging up completely and the engine can't expel exhaust gas and it stalls out.
One thing you can try here, is running synthetic oil. I know a LOT of people that have done this and noticed a smoother, quieter ride overall. Most are automatic vehicles though but that shouldn't matter. I gather you shift in the 2500-3500rpm range with the smaller engine. I know I would, as it cruises at 60mph around 3000rpm to my calculations, yes?
As you mentioned 'low rpm and high load', which I interpret by 'load' you are referring to one of two things. You have extra bodies in the vehicle with you and are trying to get the best fuel economy, or you're lugging the engine by putting the pedal down while the engine turns over a mere 2000rpm or less and takes a while to gain speed.
Curious, have you changed the gear oil yet? Perhaps adding a slightly thicker formulation will help you out a bit, along with synthetic engine oil to better allow lower engine speeds under load. Just a thought!
'00 Durango R/T 360ci 290hp (modded); 138,500m
'06 Pontiac G6 GT 3.5L 220hp; 44,000m
'12 Chrysler 200 Limited 3.6L 283hp; 13,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!