2001 Pontiac Bonneville sucking noise under the hood

Hi 26 mpg is a great avg for this model. I also have the same model. The sucking sound your hearing is a vacum leak most likely near the front and top of your motor. Mine had a similar leak, and I found it to be at the two plastic hoses that go in to one small length hose at the air intake chamber right in front of the chamber leading into the super plenum. Thats the large duct work coming from your air filter. The other smaller hard plastic hoses also tap in to rubber connections that may also be leaking some. Be careful and check the small hose at the far left looking at the motor, its a very small horseshoe shaped hose. It may be very brittle since it sits so close to the front valve cover. However if you break it, just get a longer piece of gasoline hose and use some blue silicone sealer for the connections and slide them back together. You'll be good to go then. Use this process on all your hard plastic hose / rubber connections. After the blue silicon sealr cures you will have a much better sealed connection. I hope this helps, peace to you and good luck.

Hi 26 mpg is a great avg for this model. I also have the same model. The sucking sound your hearing is a vacum leak most likely near the front and top of your motor. Mine had a similar leak, and I found it to be at the two plastic hoses that go in to one small length hose at the air intake chamber right in front of the chamber leading into the super plenum. Thats the large duct work coming from your air filter. The other smaller hard plastic hoses also tap in to rubber connections that may also be leaking some. Be careful and check the small hose at the far left looking at the motor, its a very small horseshoe shaped hose. It may be very brittle since it sits so close to the front valve cover. However if you break it, just get a longer piece of gasoline hose and use some blue silicone sealer for the connections and slide them back together. You'll be good to go then. Use this process on all your hard plastic hose / rubber connections. After the blue silicon sealr cures you will have a much better sealed connection. I hope this helps, peace to you and good luck.