DIFFERENT ENGINE NOTE???

[ED_MX5]

I'd say you ma find they replaced air filter and plugs as has been mentioned. Air filter, depending how dirty old one was, will have much less restriction. This allows more air to pass, that will change exhaust note.
Spark plugs can change it too, if they were really worn before, new ones will make a difference because engine does not have to labour quite so much to produce power. That will change where the engine starts to drone. Another possible one is mixed with other two, fuel filter. Simply because most people forget about it gets so clogged I've had some fuel filters when I was a mechanic where the fuel came out black. So suddenly increasing fuel + the other bits in tune will make up a change in noise.

Hopefully that helped you understand the 'WHY' behind the noise change.
I know the feeling btw, I have had exact same thing with a car I once bought. It completel changed noise after a full service, luckily it changed for the better. Had such a nice note.

[OR31LLY]

I'd say you ma find they replaced air filter and plugs as has been mentioned. Air filter, depending how dirty old one was, will have much less restriction. This allows more air to pass, that will change exhaust note.
Spark plugs can change it too, if they were really worn before, new ones will make a difference because engine does not have to labour quite so much to produce power. That will change where the engine starts to drone. Another possible one is mixed with other two, fuel filter. Simply because most people forget about it gets so clogged I've had some fuel filters when I was a mechanic where the fuel came out black. So suddenly increasing fuel + the other bits in tune will make up a change in noise.

Hopefully that helped you understand the 'WHY' behind the noise change.
I know the feeling btw, I have had exact same thing with a car I once bought. It completel changed noise after a full service, luckily it changed for the better. Had such a nice note.

your last point there is spot on!!!
Sounds like I half have a CAI installed, but thats still sitting in my garage waiting to be installed