The whistling noise is a direct result of air being pulled through a small gap or restriction. In this case, it’s happening at the filter rack. When air moves through tight spaces at high velocity, it creates turbulence, leading to the whistling sound. The fact that touching the filter stops the noise suggests that the filter is slightly loose or not fully sealed in place. Here are the most common reasons this happens:

• Undersized or Loose Filter
  • If the filter is slightly too small for the rack, even by a fraction of an inch, air can bypass the filter through the small gaps, causing a whistling effect.
  • Pressing on the filter might temporarily close those gaps, which is why the noise stops when you touch it.
• Warped or Bent Filter Frame
  • Some filters, especially cheaper or thinner ones, can warp slightly due to air pressure over time.
  • If the filter isn’t sitting perfectly flat, air can sneak through small openings around the edges.
• Improperly Seated Filter
  • Sometimes, the filter doesn’t fully seat into the track of the filter rack, creating a small gap where air can rush through.
  • This can happen if the filter is slightly too big or if the track has dirt buildup preventing a proper fit.
• Filter Rack or Housing Issues
  • If the filter rack is slightly bent or misaligned, it won’t hold the filter snugly, leading to unwanted air gaps.
  • Some filter racks have flexible clips or foam gaskets that wear out over time, making it harder for the filter to seal properly.