09 HHR SS manual 118k miles. All stock with exception of GM stage 1 done about 100k ago. No repairs to motor at all. Last 6 months or so I have noticed a high rate fluttering noise under WOT once boost hits about 13PSI and higher. The rate of the noise is around 5-10 beats/sec. Seems to be coming more from drivers side but not really sure and I need to get someone to listen from passenger seat. Any ideas where I should start?