That's because F1 2018's BDL files have the same sound duplicated for both the actual engine sound and the sound coming out of the exhaust. F1 2018 ignores the exhaust sound, but F1 2017 makes use of it, so if your engine sound is duplicated, you will have this weird effect.
I explained how to fix this to someone else, so I'll copy-paste it here. The gist of it is that you have to combine the raw engine sounds from F1 2018 with the exhaust sounds from F1 2017, and the process is as follows:
-Open the desired F1 2017 and F1 2018 BDLs in a hex editor (HxD in my case)
-Search for the string "egoeq" in the F1 2018 BDL - when you find the third entry, that is where the exhaust sounds start
-Once you've found the third "egoeq", highlight the entirety of the data from the first E in egoeq until the end of the file and delete it - keep your cursor here on this file as that's where the data you will copy soon will be put in
-Next, open the F1 2017 BDL and locate the third egoeq entry in that file
-When you find that third egoeq entry, highlight the entirety of the data from the first E in egoeq until the end of the file and copy it (CTRL-C)
-Go back to the F1 2018 BDL and paste the data you just copied into the file
This should fix the weird audio artifacts.
I explained how to fix this to someone else, so I'll copy-paste it here. The gist of it is that you have to combine the raw engine sounds from F1 2018 with the exhaust sounds from F1 2017, and the process is as follows:
-Open the desired F1 2017 and F1 2018 BDLs in a hex editor (HxD in my case)
-Search for the string "egoeq" in the F1 2018 BDL - when you find the third entry, that is where the exhaust sounds start
-Once you've found the third "egoeq", highlight the entirety of the data from the first E in egoeq until the end of the file and delete it - keep your cursor here on this file as that's where the data you will copy soon will be put in
-Next, open the F1 2017 BDL and locate the third egoeq entry in that file
-When you find that third egoeq entry, highlight the entirety of the data from the first E in egoeq until the end of the file and copy it (CTRL-C)
-Go back to the F1 2018 BDL and paste the data you just copied into the file
This should fix the weird audio artifacts.