Loki84

Hi everyone, I'm just wondering if anyone could give me a heads up as to what might be going on with my '99 Echo. Every time I acclerate it makes a high pitched whirring/buzzing sound. It stops when I take my foot off the accelerator. My mechanic pulled the alternator apart last time I had it serviced (last month) and told me that was fine (that was also making a strange noise) so I'm guessing it's nothing to do with that. Any ideas or solutions would be great!