Exhaust smell

Hi, My 2000 '93 interior fills up with exhaust fumes when standing at idle when the heater fans going, even for a only few seconds. My 2001 '95 never had this problem until recently but now in a relatively short period has the exact same problem so I'm assuming this might be a common issue for Saabs around this period of manufacture. Both are nearing 90,000km or 56,000miles. Is this due to something 'blocking' and needing a good clean or replacement of parts? I want to help my mechanic locate the problem witout looking around for hours without resolution. Thanks, . . . . .Eddy