Engine sound changes when car gets warm? Tb48 - read details inside

[shadzketb48]

So as of late, after about 20 minutes of driving and the car gets warm, the tone of the engine changes completely, the toughness goes away and it sounds more like a 8 ton truck or something whining , exhaust sound changes completely and you can notice a power loss, unable to get a video right now as its in the shop getting diff gears swapped, but it goes from sounding like a tb48 to sounding like a bodgy Mitsubishi magna lol.

I've got a new intake pipe coming I'll fit as soon as it lands as I think it may have a crack or a hole due to the age + clean the throttle body while im at it, I'm cleaning the mad sensor on a maybe 2 week basis and it seems to help but not fully. Any ideas where to go from here on boys? Starting to do my head in..

Some updated info about car -


260,000 ks ( just had an engine reco as it was on gas + fit new billet oil pump gears - this done less than 1,000ks - 2 months )
Straight petrol
Pacemaker extractors + 2.5 custom mandrel bend exhaust
4 inch snorkel + airbox



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