Insect in Outside Wall

I realize this may be somewhat off topic, but I can't find another group that would handle this type of issue on usenet. I've also googled extensively, but haven't found anything worthwile (other than beehive removal, which I don't think applies here). So I hope it's OK to post this here. I hope to PREVENT any extensive home repairs at any rate. ;)
I live in a concrete block building in Florida. In the past few days, I've heard a buzzing sound about twice a day inside the one outside wall. I only hear it at about 8:30 a.m. and again at about 2 p.m. Other than that things are quiet. I've already called in a bug expert who specializes in bees and such. He seems to think it's a solitary wasp that is getting in somewhere, but he couldn't seen where it may be entering the house.
I have spent some time outside monitoring the area and haven't seen any activity. He suggested that he could "dust" the area, but that it wasn't really worth him charging me for it (he checked the problem out for free, but would have to charge a service call if he did anything -- he was very nice about this, but not nice enough to tell me what I could do myself ;)). This leads me to my question: What can be done here? Is there a particular product that I could dust the general area with myself? Should I drill a hole in the wall and spray or dust inside it? (I can't pinpoint the exact location between the studs though, so it may require several holes) Or is there a kind of bait that would work and also serve to destroy any potential nest? If so, where would I best place it?
Since I can hear the buzzing sound quite clearly, I assume that the insect is between the concrete block and the drywall. And since I only hear it for short periods of time, but on a semi-regular basis, I assume that it is indeed a solitary insect (at least so far). Of course there was no buzzing when the bug guy was here, so he never did hear it.
Any help would be greatly appreciated! KB
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pound or hit wall with fist to check occupancy, basically wrap on door say hi...
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:) I have spent some time outside monitoring the area and haven't seen any :) activity. He suggested that he could "dust" the area, but that it wasn't :) really worth him charging me for it (he checked the problem out for free, :) but would have to charge a service call if he did anything -- he was very :) nice about this, but not nice enough to tell me what I could do myself ;)). :) This leads me to my question: What can be done here? Is there a particular :) product that I could dust the general area with myself? Should I drill a :) hole in the wall and spray or dust inside it? (I can't pinpoint the exact :) location between the studs though, so it may require several holes) Or is :) there a kind of bait that would work and also serve to destroy any potential :) nest? If so, where would I best place it?
I don't think you should just drill and treat without knowing what you have. Guessing it is a type of mud dauber, which are harmless, and treating with a dust should stop it but you need to find the entry way and make sure what you are dealing with.
Lar. (to e-mail, get rid of the BUGS!!
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Try bee trap/bait and put it outside near the buzzing wall. Hardware store sells them or you can buy online.
I saw on "this old house", one exterminator use a non-contact thermometer to locate colony of bees inside the wall since they warm up the wall a little.
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Thanks Peter! I wasn't sure whether they made bait specifically for bees and such, and the local hardware store was no help. I'll check it out online or perhaps one of the smaller hardware stores can help me.
Also thanks to Lar. You're definitely right about not just drilling away. That probably wouldn't have been very bright on my part. ;) The exterminator also checked to see where the insect might be entering, but also couldn't tell. I had recently (about 4 weeks ago) sealed a minor gap between two roofs, but he assured me that there was no way the insect could survive that long, and of course I am enclined to agree. If the bait doesn't do it, I'll have to camp outside and see if I can figure out where it's entering.
Thanks again to you both!
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clipped

Very, very strange that you should hear it at such regular intervals. Got an old battery alarm going off somewhere? Computer in room set to do something at those times? :o) We occ. get wasps building nests at top of window recess and inside hurricane shutters, but they are outdoors and easy to see. You should be able to see nests or entries outdoors, if it is wasps. I don't know a Florida bug that buzzes inside walls. Yellow jackets like to build inside walls, but there should be a crack or crevice where they exit and enter. No sign of dead bugs or bug parts, like wings, inside (on windowsills, around doors)?
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It's not totally regular, e.g. I heard it at 12:30 yesterday, and at about 2 p.m. the day before. But it's right around those times, once in the a.m. and once in the early p.m., and I can only hear it twice a day. Definitely not an old alarm, computer task or any such thing. ;)

I get quite a few of those, too. Mud daubers and solitary wasps that build paper nests (?). Never had one inside, but something is definitely buzzing inside that wall!

No debris anywhere, no bugs inside the house, no visible crevice, other than a small space between a vent plate and the soffit that I sprayed today and will seal tomorrow (part of the porch's flat roof though, so I don't think there's anything that connects that particular part to the rest of the house, as the roof ends at the concrete wall). Other than that, everything within about 10 ft. of the wall looks sealed. Like I said, even the bug expert couldn't see any issues. I'll definitely keep an eye out for debris though. Maybe that'll lead me to it. Thanks for the hint!!
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