Strange particles from air vent and return air duct sealant question


I've got this problem in my house, it is a 1 story with central air and gas heating system by Lennox. The other weekend, I noticed that there are small tiny brownish-reddish color particles all over my computer monitor. My computer monitor sit directly beneath an air vent. Because the monitor is directly beneath the vent and also because the color of the monitor is white, therefore, I can easily see those tiny particles.
I open the air intake filter door (20 inch X 30 inch), the one that is in the wall, took out the air filter and look behind it. I noticed there is a big dead cock roach in it. I then vacuum it out. Within the next day, the amount of those strange particles which fallen on top of my computer monitor has significantly decreased. It has been one week since I got that roach out, now I only see two or three tiny flake like particle each day on top of my monitor. Before I remove the roach from the air return duct, there were a lot of those strange particle. This happens during both air conditioner or heater operation.
I just hope that those small particle is the roach's body parts like the wings etc... I also hope that it is not rust blowing from my vent. Could it be possible that rust can be blowing from the vent some how?
It is hard to tell if I see those particles blowing out of other vents in my house because the only reason I noticed those tiny particles in that particular room is that my white computer monitor sit directly beneath the vent and that vent is about 5 feet away.
When I look in that space area behind the air filter, I noticed there are some gap/tiny cracks and I want to find ways to seal it so that no other insects will crawl into it in the future.
My questions here are:
1) based on the problems and symptoms I described do you think those particles is from that roach or other insects may be stuck somewhere in my AC/heating system or is it rust in my system that is blowing out of the vent?
2) How do you seal these area in the air return duct which is behind that big air filter on my wall? What to use (joint compound, caulk, foam sealant etc..) a) small gap where the drywall meets the concrete floor b) tiny vertical crack at the corner where two vertical dry wall meets. c) Where the 4x2 wood meet the foam insulation sheathing.
Thank you very much in advance.
Jack
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Worst Case: it might be a failed heat exchanger, so gas combustion products like soot etc is getting into the exchanger part of the air circ. circuit. May need a new part, or a new heater console. This is a dangerous condition as it could introduce CO into house. Best Case: Could be particles of rust from exchanger, or from duct walls. Perhaps someone hit or vibrated the ducting at some point, loosening the rust.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.