[wiswind]

I am not good at tracking down sounds......but some sounds are normal.
I have a '96....so I am not an authority on sounds in your '99.

The 2 vaccum regulators on the back.....if they are the ones that I am thinking of....have an electrical connection as well as a vaccum connection.

One is the EGR vaccum regulator solonoid, it converts the electrical signal from the computer to a vaccum that is applied to the EGR valve.....
If it is not working, you will get a CEL and code relative to the EGR system. Although the most common cause of a EGR flow code is the DPFE.

The one on the firewall, may be the fuel system canister purge solonoid. It takes the electrical signal from the computer, and purges the fumes from the charcoal canistor. There are failure codes for that, if it is not working.

Another possible vaccum regulator is for the IMRC, which opens the "butterfly" for each cylinder above 2900 rpm. The butterflies are open with the engine OFF......and you most likely could see them when you were replacing the isolator bolts....they are in the aluminum lower intake manifold. When you start the engine, they close...to give you better gas mileage.....and then they open when you are above 2900 rpm, to give you more power.
There are fault codes for those, if they are not working as they should.

My suggestion....would be to see how the car does for the next while...and see if any fault codes come up. If you have a significant vaccum leak, it will throw a code and CEL.