BMWs have a rubber boot between the air filter and Mass Airflow meter which can split, leak, and whistle. Some would replace it. Others stuff a pipe into it.
This 16 year old Volvo was running rough because the original fuel filter was restricting fuel flow. Fuel filters should be replaced every 2 years or 30,000 miles.
"Car smells bad" often means mold in the AC evaporator, a difficult and expensive repair. In this case it was simply a moldy 'Cabin Filter"
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