Hello, My name is Mike and I'd like to help you with this problem. Since I cannot hear it , and you can, I'll give you a few things to start with and hopefully we can reach a conclusion. We need to find out first if this is an internal engine noise or something external that might be easier to repair. First, Make sure you have done all of the normal things like check the fluids and look at the belt.
If the engine is still cold, grab ahold of all of the thin heat shields on the exhaust manifold and the exhaust pipes under the car to see if you can find anything loose.
With the engine running, and while the noise is present, move around the engine compartment and kneel under the car in a few spots to try and narrow the noise to where it is coming from. Listen real carefully to areas like the exhaust and other external noises.
There are a lot of esternal items that can rattle more at lower rpms under load. This is because your engine has more natural vibrations under this low rpm/high load condition.
If efforts to locate any extrnal rattle fail, you may have an internal engine issue. This will require a more experienced ear and some disassembly, or removal of the engine.
I hope this answers your question. If so, please click accept.
Thank you,
Mike
ASE Certified Technician
20 years experience, all makes and models.