I agree with Jay J. I to have a similar scenario except my vent piping is behind drywall. The method that I used to help clean out my venting problems was two fold and afforded me some success. First use a garden hose with the end capped off and snake through the vent from both the outside and inside. Second, if you have access to a leaf blower use it to blow from the inside while using the garden hose from the outside. Hope this helps, and maybe you won't have to get under the house.