Engine noise usually has to do with three things, engine balance, exhaust tuning and intake tuning. Sometimes other factors are involved, such as valve train, valve timing system, etc.

I'm an old Ford fan so I think my comments are fair, but I've driven my fair share of Ford 4-cylinders of the past and most of them have not been particularly smooth or quiet. Between exhaust note, valve system noises and general vibration, they are solid engines, but not quiet or polite. Ford has done a nice job improving their 4-cylinder engines, but when I purchased the Prius, I was so surprised and impressed at how different The Toyota 4-cylinder behaves compared to older Ford 4-cylinders I'm used to. Less noise, much less vibration and much more polite overall. As a side note I find it very interesting that the I-4 that Ford is selling in the Fusion/Milan is a 2.3L like the old Tempo/Topaz series. Although I'm certain it is a very different design. However, over the years 2.3L has been Ford's favorite displacement for 4-cylinders along with 1.8L and 1.9L for Escort and Focus.

Anyway, the hybrid drive system that Ford uses is quite similar in construction to the HSD system Toyota has, specifically with the use of a power split device and 2 motor/generators. Simply put, engine RPM is not related to ground speed but is related to what the computer deems necessary to maintain the ground speed that you are requesting with the throttle. This means that unusually high RPMs can often times happen when you least expect them, at least according to conventional logic. There is some conventional logic which kind of has to be thrown out the window with hybrids of this mechanical design.

As for rollover: Ford has been real aggressive in designing that out of their vehicles, at least as far as engineering can take them. User error is still an important and heavily glossed over issue with regard to vehicle rollover. My take on the 28psi/Firestone incident is that the factory tire inflation spec was too low and to make things worse, factory rubber is notorious for being the most inexpensive rubber you can put on the shipping rims. Being that Ford was using Firestone rubber, it was probably the worst rubber they could get away with and Firestone was happy to sell it to them. Just talk to Prius drivers. Most of us are not happy with the factory rubber. I'll be really glad to get rid of mine when I believe they have sufficient wear to not be a waste of money to get rid of. What I find so interesting about the rollovers is that in general the Explorer has the lowest center of gravity of any SUV built up to and during the time of the rollovers. Mudders and outdoors enthusiasts always ragged on the Explorer as being a pavement 4wd, which it was. I was not inclined to believe that the vehicle was at fault or that there was a design flaw in the suspension. It was definitely related to the factory spec tire pressure and the Firestone rubber. Now, the question is who was at fault? Both. Firestone knew that Ford was specifying a very low inflation pressure. Ford knew that they were putting cheep rubber on the Explorer. A conglomeration of errors.

It has been said:
Hybrid drivers come in 3 flavors, greenie, techie and cheapie. Pick any 2.
2005 Prius, Melinium Silver over gray, package 5 (AI)