Of course the sound was to change, because the engine is now in a different state. It probably sounds like a new idsi engine since you have been hearing to it in a bad shape since quite some time.
acha, by the air filter thing, do you mean the air box was broken? or was the cng venturi lose? Cz I am not sure how can it suck so much dirt inside just due to a leak.
and more than sub standard coolants, what actually damages the pipes is plain tap water used in the radiator. Even that doesn't harm the pipes that much k they start leaking, what actually happens is our mechanics remove the heater water distribution piping to improve AC performance in summers. It leads to pipes drying up and that promotes rusting. I am sure that has happened in the history of ur car otherwise there is no way the pipes could've rusted to death and you wouldn't even know.
And I have a idsi 2003, it has 218k kms on the clock and is MashALLAH running more than perfect. Though I don't consider it very reliable as it has already lived its life. Its just the way it performs that makes me count on it everyday.Though soon inshALLAH, I won't be over hauling the motor, I'll simply swap in a L15A vtec motor in it. More power, more economy.
@ahmadbila has 2006 idsi having 260k on the clock and mashALLAH, its even better than my car. AT the end of the day, maintenance is all that matters. Carry it out in a decent way and your car won't be giving up on you for at least 10 years.